I spent about 4 hours playing Friday night, about 3 Saturday, and another 3 or 4 on Sunday. Amazing game. I could never get into Morrowind because of the obscureness of the primary quest, but I've been drilling through the quest in Oblivion fairly quickly I think, although that may be a bad thing, since I'm a lot farther along than people I talk to on IRC who are levels 8-15, and I'm still only level 3, so I guess I'll have to start taking on some side quests and visit the arena.
Specs:
Opteron 146 @ 2.8GHz
2GB RAM
DFI Lanparty Ultra-D
eVGA Geforce 7800GT
The graphics are stunning. The game put me at ultra high settings at 1024x768, but I bumped it up to 1280x1024, which is the max on my 19" LCD, and it runs great. The UI is a little overly large, no doubt due to a direct conversion over from the XBox 360, and I've had a few annoying issues that were due to the Radiant AI not responding the way it should have (rode my horse into Anvil, left it on the boardwalk to go into the castle, came back, jumped back on, and was arrested...figures), otherwise, I'm thrilled with this game and play to play it as long as it will hold my interest, which hasn't been an issue so far.
The sheer size of the map is stunning, and it takes a long time to run from one point to another, which makes the instant travel option a godsend. I've found probably a dozen random ruins and dungeons just running places for quests I have. The game is massive, there's no doubt about that. I'm sure for a pretty casual gamer like me, it will last at least 4-5 months before I start getting tired of it, and with a bunch of mods already starting to come out, probably not even then.
I'm getting an Option 165 tomorrow to replace my 146. Be interesting to see if I notice anything, of course the 165 probably won't run at 2.8GHz like my 146 does, so I might not be able to make a valid comparison.