I own all three games...
Discworld - I had the PSOne version originally and got to around the 1/3rd mark only to discover that my second hand disc had a nasty scratch on it, causing me to be forced to give up. Years later I got the PC version through dubious methods (can you blame me after wasting my money on a legit copy) but I've yet to play it.
I'm probably going to plunge in sometime this year via ScummVM. Anyway, from what I played I thought it was good fun but I'll reserve judgment until I've payed the whole game.
Discworld 2 - I found a copy at the charity shop I was working in at the time (CD case with no box or manual) so I snapped it up before any customers could get their hands on it (which is one of the fringe benefits of working in a charity shop). I played through the whole thing without the use of a walkthrough and whilst I found the graphics to be beautiful, the story and characters to be charming; the puzzles were completely infuriating.
Seriously, play this game for the first time, without a FAQ guide like I did and you're liable to go insane. Having said that the feeling of accomplishment when you do solve a puzzle after hours or days of trying is immense (or at least until you realise that you've just spent hours or days wasting your life).
Discworld Noir - Again, this turned up in a donation to the charity shop. I've never got it to work under Vista and according to the Internet; I'm not alone. I can only imagine that that was the reason it was donated in the first place. Still, I'll hang on to it in the hope that a fan might release a patch.
So in summary that's my experience with the Discworld videogames; 50% frustrating gameplay and 50% technical headaches. Pretty good, I'd recommend them!