Have you tried reading the manual?

Only you can decide that for yourself. Whenever I want to decide if I'll like a game I watch one or more Let's Play videos, which will show off the gameplay, load times, possible UI difficulties and anything else.

Try watching LPs for MM3. If you don't like the gameplay or graphics, jump up to MM6 and watch some LPs for that. If you don't like MM6 either, then skip the series. As has been said, MM and HOMM are different games. The MMs are CRPGs, while the HOMMs are fantasy strategy. If you like HOMM 3, I recommend the WOG or ERA mods.

There is a storyline that weaves between HOMM 1-3 and MM 6-8. This is the nature of CRPGs, the ability to explore the world however you want and do whatever you want. More recent CRPGs do a lot more hand-holding with the player, telling you exactly where to go and what to do. Older CRPGs give you quests, but let you decide when to do them and sometimes even how to do them.

If you love to just explore new worlds and poke around, always eager to see what's around the next corner, CRPGs are for you. If you want more linear "do this, go here, now do this" type games, CRPGs might not be for you. RTFM Only you can decide that. The replay value of CRPGs consists of doing the same quests and areas with new parties, or new strategies, or roleplaying differently. There is no new, randomized content.

Maybe someone needs to write a randomizer for one of these games?

Actually, this also depends on what style of CRPG you're looking at. WRPGs typically (not always) are less linear and let you do whatever you want (within what the game mechanics allow, of course). JRPGs typically (not always) are more linear, requiring you to do a specific set of tasks in a specific order in order to progress the game (though sometimes there are sidequests).

The Might and Magic games, at least the ones I've played (which include 2 through 5), with the exception of Swords of Xeen (which seems to be intended to be linear, but there are bugs that can be used to circumvent that), fall into the non-linear camp. (I should note, however, that MM5 felt a bit more linear than the others; in particular, the later part of the game has to be done in a specific order, and can't even be attempted until you've collected a certain number of a certain quest item.)

As others have pointed out, first discard any thought of Heroes of Might and Magic, those are entirely different games, nothing to do with Might and Magic (other than some storyline connection between some, but referring to gameplay).

Also as others have pointed out, the Might and Magic games are massive, and massive time sinks, taking hundreds of hours each. If you want to go through the entire series, 1 to 9, at four hours per day, every single day, I'd say it'll likely take you over one year. So, referring to the replay value, if you'd still want to replay after that, be my guest.

About the games themselves, yep, reading manuals is important, but don't shy away from on-line guides either. In fact, maybe you can find out the rest yourself as you play, but I'd say it's crucial to have the character development worked out from the beginning and the plan in front of you as you play, with how to maximize development, all the tricks and secrets, so you won't miss anything from that point of view. Can make all the difference between enjoyable games and untold frustration, or even ending up stuck, impossible to continue.

About the games themselves, here I'll just make this opinion be worth quite little since I just played 6, and that was around 2006, and a bit of 7 before giving up back then, and a bit of the first one more recently. With the character development worked out and followed, I'd still say M&M6 is a great game which will stand the test of time. If that one doesn't pull you in right away, I'd say the series is not for you at all. At the same time, if you start with that, the others will likely be disappointing.
On the other hand, the first 2 (actually just these days looked into 2 more again, checking guides and watching videos, before deciding to not give it a chance after all) are very archaic, with interface and controls that were probably poor even for their day. What really got me was saving just at inns, couldn't deal with that. Also no in game map in the first, but there are great maps you can find on-line. Otherwise, for me all the keyboard only controls and the keystroke sequences required for every little action were nuisances I could deal with, but for more... modern gamers they can be baffling and maybe even too... weird to manage. Spells were one big nuisance though, with no in game info, needing the spell list and details written or printed out since you just type spell level and number to cast and need to know what each means.

I would agree 6-8 are the best combination of quality and accessibility.

World of Xeen (technically M&M 4: Clouds of Xeen which combines with M&M: Darkside of Xeen to make one mega-game) is maybe the most ambitious, and worth going back to.

Technically there is also the standalone Might & Magic: Clash of Heroes, but other than being in the same universe really has nothing in common in terms of gameplay.

No.

I would like to mention, that M&M VI (and older) do not have deep specialization system, so if you can learn some skill you can be best in it (e.g. in newer games, you can learn something, but never become expert etc.). On the other hand there are some mods which can add new blood to old story and some are still in preparation. Either way I can recommend M&M and there is no need to fear about replay value, anybody who finished some of them can confirm it. ;)

The introduced specialization was one thing that pushed me away from 7. Really wanted to keep the character development freedom of 6.

I would like to mention, that M&M V (and older) don't have the sort of skill system that is found in later games. With the exception of Thievery, you either have or don't have each skill (there's no such thing as being better at the skill than another character), and everyone can learn every skill except Thievery. (Well, in MM3 and Swords of Xeen it is *possible* to teach Thievery to everyone, but they lack the bonus that Robbers and Ninjas get, and you can't do this in World of Xeen anyway. I probably should mention that Thievery doesn't do much in Swords of Xeen, and the Swords of Xeen is definitely *not* a good introduction to the series.)

In MM2, each character can learn up to 2 skills (there's a way to delete unwanted skills), and Thievery is a stat rather than a skill. (There do exist ways to get non-zero Thievery for non-Robbers/Ninjas, but they involve specific equipment, and any way to get it up permanently (there are a couple methods to my understanding) are the sort of things many players would consider an exploit. Then again, there is a puzzle where the intended (and only, to my knowledge) solution involves something reasonable players would consider an exploit.)

In MM1, there are no skills at all; class, level, and stats are the only things that differentiate characters. (I don't think there's any notion of spells known; any character with access to a spell level can cast every spell of that level.)

*nods* Just need to reach the required level in that class.

Actually, to my understanding, what matters is your Spell Level (SL), which is usually determined by your level, but might be temporarily changed.

I know that MM2 has some effects that can change your SL, but it's not quite as useful there because you actually need to learn the spell. (It's worth noting that hirelings start knowing all their spells (including those of a higher level than they would normally get access to for Archers and Paladins), and taking advantage of this can be hugely beneficial.)

(In at least the DOS version of MM2, there actually is a way to get non-casters to cast Cleric spells.)

You are really an encyclopedia of games like that. Do you keep notes or do you have all that in your head?
No matter what, it's impressive. I was really surprised you didn't know Dark Heart of Uukrul though in one of the others threads. It's probably exactly your kind of game.

Not as bad as the manual needs from the D&D classics i hope?

Something like 'You hear some rumors in the bar. Read page 22, #1 #2 #7 and #9' or something like that.

Literally the game was unplayable without the manual. Worse the manual was a pdf file on a CD i had, and alt-tabbing was too annoying.

Oh, definitely worth playing. M&M games are medium in terms of "blobber" difficulty. They're also incredibly goofy (especially M&M 3-5), if that's your cup of tea. However, they nail the exploration part, even if the combat is not the best in the genre.

IMO the difficulty is variable. 1 and 2 are difficult, 3-5 are easy.