For me, the one that was the most disappointing was Gothic 3 not because of the game itself but because I never got it to go beyond an empty black screen while it throws out error statements. I did get it on sale, but I still never got a chance to even try the game.

I need to change this one. GOG.com made changes to the downloader (the game is now updated with the community patch) and the game runs fine. I need to change my answer now. I'd have to say Ishar (although I do like it a little) is probably the most disappointing. The controls are difficult, and the game is too hard to be enjoyable because of this. It's not my least favorite (that would be Might and Magic I- but that one came as part of an excellent package also including II-VI) but I just can't really get into it.

Got an ATi card?

There are some titles that are problematic for me largely for technical reasons:

Starflight 1&2- Numerous problems (frequent crashing, easily corrupted saves, and major game speedups during realtime sections)

Disciples- Even after getting the game to run, it will frequently crash after awhile

Zafehouse Diaries- Text based game with tiny fonts that are very hard to read on my laptop. Super random events & resolutions makes this game unfairly difficult.

Other than buying game full priced and having it discounted soon after, I don't think I've had major disapointments yet. Number of games have been "meh" but that's to be expected. There are totally shit games in catalog (MoO 3, Star Control 3 among others) that aren't worth a penny but imagine my disapointment with them when I paid full retail price (~50€!!!) for them back in the day. At least I learned my lesson: "Never buy anything upon release, always wait at least three months before buying.".

I just remembered I also regret Heroes of Might and Magic V (though not to the same extent nearly as Invisible War) and I am so glad I got it on sale. It's not the worst, but honestly, it doesn't hold a candle to Heroes III, but then again, few things do, that was kind of a high point. I'm glad I didn't buy V on release, unlike with III.

I've not had any GOG purchases that really disappointed me (a few of the games I got for free aren't too good). I was a little disappointed with (*gasp*) Arcanum when I first started, especially after all the hype I'd heard. It does have a number of flaws and in the beginning the main story line looked like pretty clichéd stuff. However I am getting into it much more and now I wouldn't say I'm disappointed.

None of the games have really disappointed me. A couple I got on sale and ended up not touching them until they had come along at a bigger discount, but those few dollars aren't worth losing sleep over.

If I had to pick one I'd guess Retro City Rampage. Its not like the game is bad, just kinda OKish overall to me.

Fahrenheit is one of the ten worst games that I have ever played.

Ring Runner is unfunny and tedious.

Zafehouse Diaries is too random for my liking.

I'd have to say Ring Runner was my only let down.

Was looking for an rpg/flight hybrid type deal, but was just to simple.
Saw that it had customizable ship.. but takes forever to save up to buy all the pieces.
And I never could get multiplayer to work... which I'd probably have a disadvantage because I had low tier ships/parts.

Been a few months since I've played it, maybe an update can unlock all components/ships/parts for multiplayer at least.

Disappointed no, but i bought Blitzkrieg and didn't saw that its a RTS....
But on the brighter side, i haven't bought Sacrifice ;-)

Soul Reaver 1, Soul Reaver 2, and Blood Omen 2. Not that they're bad (well Blood Omen 2 is kinda bad) they just don't want to work on any of my computers except for my ancient Windows XP machine.

I haven't tried most of my purchased GOG games yet, but of those I have, a few pop into mind right now:

1. Race the Sun: it seems even simpler (in a bad way) than what I thought. I originally got interested as the screenshots reminded me a bit of two ten years old freeware games:

http://www.bruneras.com/games_swboy.php
http://www.bruneras.com/games_swboe.php

Different genre I know, but still, I guess I was expecting more. Well, at least Race the Sun reminded of those old freeware gems, so I downloaded them again and have played them quite a bit since then.

2. Jack Orlando: it just seems so generic and average detective adventure game, boring. Not sure if I'll ever try to finish it. I would give it a benefit of doubt, but the reviews suggest my early impressions will probably remain rest of the game.

3. Theme Hospital: I'm ashamed how boring and pointless I find the game, considering the praise it has received. While I love e.g. RTS game, maybe this particular genre (some kind of management simulation) is just not for me. I even find the humor in Theme Hospital pointless. "Fat Assitis", yeah I get it, funny sounding disease, ha ha.

That's too bad, I don't recall having any real problems to get them to work on a Windows 8 machine when I tried them out earlier (a laptop, even). Well, Soul Reaver 1 has some issues with modern gamepads, but I think it is because it is designed for non-analog gamepads. So it should be played with the keyboard, unless one wants to reassign keyboard keys to the gamepad with XPadder or whatever.

Blood Omen 2 and Demon Stone.

I would want to put Enclave up there too as it's almost as bland, generic and boring as the other two but somehow it kept me going until I completed it. 5 years ago I would have enjoyed Enclave but the other two right out sucks.

Sanitarium is easily my most disappointing purchase here. I bought it because I enjoy horror and I love adventure games, and it had gotten lots of praise around the web, but I just thought that the story of the game seemed like ten five-year-olds got together and decided to write the scariest story they could think of. It was overdone, silly, and lame.

Mine are Jack Orlando, Sierra games (Quest of Glory & Police Quest) and somehow Shattered Horizon.

First for being very average, bad sound for VA, bad design, Ok with a walkthrough though..

Sierra games for being hotline whores back then, and it shows with silly design.

Shattered horizon, not really disapointing because it was part of a deal and I don't even know what it is exactly, just taking dust on my shelf and won't please me from the infos I have, 2014 I'll try and play it.. :D