So I bought Splinter Cell and Dark Messiah (yeah I know I said only one, sue me) and I couldn't be happier with either purchase. I think they are both great games (admittedly I haven't gotten very far in either one, but that's what the weekend's for!). I didn't look particularly closely at the Gamespot review of Dark Messiah but I think it was way off base and I think that shows in how different it's score is from the average.
I do know that many people have been encountering technical problems and I could understand frustration in that department, but since I haven't witnessed it myself, I can't really comment on it. All I can say is the core action is really fun, and it feels like a new take on the first person shooter genre. By the way, keep in mind it is a first person shooter with RPG elements, not the other way around. I think if you go into the game expecting something like Oblivion you will be disappointed, if you go in expecting something like Half-Life with swords and sorcery, I think you will be very pleased.
Splinter Cell on the other hand is really polished, I really only have one gripe. Why on earth would you penalize the player for incapacitating guards?! Most of the fun of Splinter Cell comes from getting the drop on your enemies like some kind of techno ninja. I understand encouraging the player not to kill anyone, but just knocking them out? Even Solid Snake is allowed to do that without repercussions. I intend to play the game the way it is meant to be played, not just sneaking around trying to avoid everyone, my stealth score be damned! As a note to the devs: we like taking out guards, don't punish us for that, it's what makes the game fun.