It very much depends on what kind of player you are and what you plan to do with NWN.
- If you only want to play the three main campaigns in single player and nothing else, you probably don't need the EE. It will only offer a few extra effects, slightly more saturated colors etc. at the cost of slightly more weird shadows on characters' faces, and possibly a few new bugs, or old ones that were fixed in Diamond 1.69 but are still present in the EE.
- If you never played the old previously DRM-d premium modules like Pirates of the Sword Coast and you're interested in buying and playing them legally now, or if you want to play the enhanced and extented versions of Darkness Over Daggerford and Tyrants of the Moonsea with voiceovers, more quests and some new graphics, tilesets, models, the EE is required and worth it, especially at the current low prices.
- If you want to play NWN online, the EE is definitely worth it, too, maybe even a must-have, as it's now much easier again to connect to Persistent Worlds. The server lists didn't work in Diamond anymore but they do work in the EE, and many PWs have been updated to the EE and are not compatible with Diamond anymore. Plus, the EE now gives PW owners the option to provide required mod files via the in-game downloader, so many PWs are click and play now, without the need for you to go to external websites and install lots of required haks manually.
- If you're interested in user-created modules, the EE now features a curated selection of very popular ones in-game, too, that you can download via the Community tab in the game menu by way of the in-game downloader/installer, also completely automatical (e.g. Aielund Saga, Eye of the Beholder adaptation, A Hunt Through the Dark etc.). I believe at least some of them also were updated to EE standard, so the newest versions might not be available for and/or compatible with NWN Diamond anymore. That being said, you should also know that while the EE is about 90-95% compatible with old modules, occasionally there are compatibility issues with some (most prominently the acclaimed Prophet series, sadly). It's very rare though.
- If you're interested in customizing your experience with mods, the EE also offers a few new advantages. You can now create your own patch haks, which is an easy way to organize your overrides files. No more dumping everything in the same override folder, you can now just write a txt file telling the game which hakpaks to load as overrides and in which order (then again, there is also
NIT - The Neverwinter Nights Installer Tool that could help you organize your overrides even with NWN Diamond). Personally, I also love that you do now have access to all 255 or so slots for heads during character creation. That doesn't change anything for an unmodded NWN, but if you have a mod with 255 different heads per race, you can now see them from the start and don't need any workarounds anymore in order to apply them retroactively. The EE also allows for other mods that were not possible with Diamond, like an
NWN2 style quickcast UI menu or an option to order around companions directly (click on them then click where you want them to go). I haven't tested these myself yet though.
The HD models are a matter of taste. Personally I don't use them as I think they clash with everything else that was not updated to HD, and don't necessarily look nicer than the originals, but YMMV.
- If you want to create your own modules, the EE is probably good to have, too. Not all but most players in the NWN community use the EE now, and the EE now includes new models and tilesets used in the new premium modules for all builders to use without having to add haks.
- Another thing you might want to consider, concerning all the people who keep complaining about Beamdog as a company that only rehashes games they had no part in creating - this is not true; there are several of the original creators of NWN working at Beamdog now, including the director of the game who was not only one of the co-founders of the old Bioware studio but also co-founder of Beamdog. So in a way, you'd still support (some of) the original devs in buying the EE.
- Last but not least, you should know that the EE is less frequently updated on GOG compared to the Steam version, because GOG only releases official patches, but Beamdog has been releasing lots of development builds with quick fixes and new features on Steam in between official patches, and official patches were very rare before, once even a whole year passed without official patch release, and during that time the GOG version was way behind the Steam version with its development builds. It has improved since then, and nowadays there aren't many big changes to be expected anymore anyway, but back then it was very frustrating. Also because when new content finally arrived on GOG, it happened that it was weeks or even a month later than on Steam. So if you only buy on GOG, never Steam, you don't have much choice anyway, but if you don't mind Steam, this is worth considering.
All that being said, unless you're only in the very first category mentioned above and you don't really have any money to spare, 3 EUR is a real steal for a game like NWN EE, even if you already own Diamond and end up being unimpressed with the EE. I think it's worth that much just for trying it out. Worst case scenario would be you just keep using Diamond and don't gain anything immediate for those extra 3 EUR you gave to (part of) the original devs. But even then you'd gain the means to play any EE exclusive content that might interest you in the future.