Hello, I'm new to GOG but I just want to say it looks like a great service. I look forward to downloading the classic games it offers. I have a little money to start a GoG collection and I've narrowed down my possibilities to either Arcanum or the Might & Magic 6 pack.
As I'm writing this, I'm leaning toward M&M 6 Pack. I figured I might get more for my money's worth, with its 6 titles in one (granted some of them are quite dated by today's standards). As long as the pack's better games are still playable and not too puzzle heavy (I'm terrible at puzzle solving ;_;) I think it could be a good bargain. It reminds me a little of the Wizardry series, presenting fantasy worlds with traces of sci-fi.
Arcanum interests me, because I believe it was made by the same people who did the Fallout games? I'm also a huge fan of steampunk, and I think there definitely needs to be more of it in gaming.
Basically what it comes down to for me is playability, stability and how many sheer hours of gameplay I can get. I need something to hold me over until StarCraft 2 :D

I know the Might and Magic 6-pack is definitely worth the price, even the dated ones are still pretty fun. (so long as you don't mind drawing your own maps for Might and Magic 1)
Although, for the first couple, you may have to turn back DOSbox's cycles count or you won't be able to read the text, and 6's character creation screen graphics are way too fast, but other than that they are perfectly playable.

Having played Arcanum several times, I recommend it is as very good RPG. Just make sure you turn the battles to turn based (you can choose real time or turn based).
It's a tough RPG at the first part due to enemies, but the game is very good if you like exploring and fighting a lot. Near the end you are near unstoppable, but that doesn't guarantee a victory at the end of the game :)
It is a long game, I can't say how long, I never took notice of the time spent on it, but if you go for side missions etc, my guess is easily 30 hours, probably more. And that is for a single playthrough. You can play as different characters (race, "profession", magic or technology allignment) and also choose to be good or evil through your choices in the game, which give replayability options of at least 6 playthroughs (IMO).
The M&M series is very good from what I have heard, but I haven't tried it yet, although I on one of the later ones on CDs. (plenty of games I haven't played yet lol)
Either way you go, you will have many good RPGing hours :)
Arcanum was indeed created by the same people (or most of them) that made Fallout 2 and it shows.
One thing that might affect your decision is that Arcanum is a 100% birds eye view game.

This is true of Might and Magic as well, although the 6-pack contains several 30-hour+ games, most of them are probably closer to 100 hours, but in some of them at least, they have the annoying tendency to start you off with no gear or money.

Yes, whatever your decision, you are getting some of the best computer games ever created.
Arcanum was for years (still is?) my favourite RPG.
In case you want to know about other rpgs, some other personal favourites that might interest you
Ambermoon/Amberstar Old, very european style graphics, very good, one of my two favourite RPGs on the Atari ST (and I think they exist on the PC too)
Phantasie III The whole series IMO is great, I would love to see them here.
Albion, the only that I know there is a PC version of, basically sci fi, but taking place in a very medieval alien world. Very good game.

Get both. Now.
Gothic I & II and also Sacred Gold are both quality RPG choices too. All very playable with very few bugs and long playtimes (particularly Sacred). Gothic is story based..very immersive and funny..good writing..Sacred I can't speak of the story much as I spent most of my time on sidequests and just killing/looting and whatnot. Combat is great anyway..quite a bit of depth.
Honestly I'd get all these games. :P
PS. Welcome. :)