Ever since I heard about the thief series earlier this year I've been interested in buying them,but I have a few questions first.

Does the gameplay still hold up?

Is the user interface fairly easy to understand?

Do they have any known problems with windows 8?

Any answers are very much appreciated. :)

I can only speculate on the Windows 8 compatibility, but if GOG lists it as compatible, then it should work. Probably best if you don't install in the Program Files folder, but that should be standard with older games these days.

As for the game itself, it sure does hold up. The graphics were fairly weak even back in the day, especially the faces and model details. However, the atmosphere is spot on. Besides, you approach most enemies from behind, so it shouldn't be much of an issue.

The gameplay is excellent, good to this day. It's all about sneaking into places and avoiding guards. You have to keep quiet and stay in the shadows as much as possible. It's not a fast game. There are some modern games that work similarly, but the difference there is that, if you're discovered in Thief, you're pretty much dead. The game is fairly unforgiving like that.

As to the UI, it is fundamentally a standard FPS UI, with mouselook and WASD movement. However, due to the nature of the games, there are other important keys to keep note of and rebind, if necessary. It's a good idea to immediately look at the keybinds for leaning left/right, inventory scroll/use/pickup/drop, environment use (e.g. open doors), run and crouch hold/toggle. There might be other stuff I'm forgetting, but you're probably getting the gist of it. A simulator this is not, be easy :-)

I don't think any of the games have a tutorial, so you'll be finding things out the hard way :P

The first Thief game definitely has a tutorial.

I can't, for the life of me, recall one, but it's probably been a good ten years since I last played it. I actually thought the third might have a tutorial, but I've never played that one.

there is a link to a free demo (which includes the tutorial) in the sticky topic.

The first Thief has an optional tutorial level, and the third one has an unskippable (if I remember correctly) one. As for the gameplay, I have to second what Spinorial said, the gameplay still holds up very well, it is slow-paced (if you don't know the levels well yet, that is) the main focus of it is stealth, exploration and observation, and combat is rarely a viable option. (Except in some of the undead infested missions in Thief Gold)

The UI is very basic, and you do not have an inventory screen, all your items and weapons can be accessed by pressing the keys you have assigned them to.

If you have the money to spend, do not hesitate to buy the games, they are well worth it.

I thought this the first time I played too, but later I learned that it's definitely possible to sneak around zombies (and spiders, but they might be harder). It's just very difficult to determine when a zombie or spider has seen you. They're hard to read. With some practice, though, sneaking past them isn't too tough, and most zombies can be outrun if they give chase.

For a first-time player, however, it will indeed seem impossible to sneak past these enemies and you'll end up doing a lot of fighting.

Even to this day it's still got the best stealth gameplay I've ever seen, and as a Windows 8 user I've had no problems with the first two games whatsoever. I've encountered quite a few bugs with Deadly Shadows, though it might not be related to my operating system.

Regarding the graphics, Thief 1 and 2 were downright ugly even back in the day. The sound engine, however, is still one the best, even to this day! And since you are mostly crouching in the shadows and moving in dark rooms, this is what really counts. Just by hearing alone you can safely navigate through the level and not run into any guards and whatnot.

Also, there is this new retexturing and remodeling project over on TTLG.com. Together with a revamped graphics engine (please look in the stickied thread above this one) this makes T1 and T2 much less of an eyesore. Also, there are certain AI scripts (that to my knowledge unfortunately only work for Fan Missions) that drastically improve AI behaviour.

Aaaand, you have a myriad of really great FMs that almost achieve the level of quality of the wonderful and immersive and beautifully narrated campaigns of the main games.

Really, if you can look past the dated graphics (or are willing to spend some time updating them with retex/remoddeling packs), you are in for a treat!

Personally I think only the character models, especially in T1, looked ugly. Otherwise the poor graphics was part of the charm of the Thief games, and help make the Thief world seem dirty and medieval. Too good graphics can make the environment seem too sterile and antiseptic, which is usually the case with CGI in movies too.