I've been "away" in San Andreas for the past few weeks (that, and dealing with a few family emergencies).  Wow.  The sheer scope of the game is just hard to believe.  The first city (Los Santos, aka Los Angeles) probably 75% the size of Vice City (though with a lesser # of missions), and there are three entire cities that size with a whole swath of wilderness between them. 

Really, it's almost too big.  I just found myself really getting to know Los Santos before my missions starting pressuring me to go to San Fierro (aka San Francisco).  Keeping all that geography in your head can be pretty daunting, and I haven't even unlocked the whole final area of the world yet in well over 30 hours of play. 

I had an interesting discussion with some other devs about "fatal flaws" in GTA that would normally tank other games.  Their tech is really aging on the PS2 -- frame rate hitches and massive pop in is common.  Their mission difficulty is crazy sometimes -- there are some very rough missions early on and you often have to wait for a particular time of day before you can try them again.  But neither of those will stop GTA from selling a skillion copies and probably being considered the crowning glory of the series (at least on PS2).  While the setting is inevitably a huge part of it, other games have done the setting without the fantastic success of GTA.

I maintain it is that sense of being in a place, a total sandbox city where you have the illusion of doing whatever you want and the game responding to it in an internally consistent manner.  It's just fun to drive around, wreak havoc, and just absorb the feeling of being "in" San Andreas.  Unfortunately no other game is even attempting that (though MMOs sometimes come close, they are rarely focused on it). 

Do I have my uber-fanboy nitpicks on it?  Sure.  I wish the radio stations were different in each city.  I sorely miss the rampages (and alas, my 3 yr old is still a bit on the young side to do the co-op rampages with me).  I liked the hidden package system better in VC & GTA3.  But really, those are minor complaints and there is so much more that SA adds that really takes the GTA experience up a notch. 

I fall into almost an "alternate life" routine ... I'm just driving around, scouting for photos, tagging gang symbols, doing drivebys and robberies, finding cool jumps & stunts, and occasionally doing my "day job" as a taxi or ambulance driver.  The missions (which are generally excellent, and more varied in SA than the previous two PS2 GTAs) become almost a sidenote, something I do to unlock new features, yet do almost with hesitation, since I don't want the ride to end.  Now, admittedly, at 30-odd hours played I'm still at like 32% completion, so there's tons to do after I finish the official story plot, but still.

Of course, there are so many other games clamoring to take me away from SA as well.  I spent many hours playing the latest Ratchet & Clank with Xavier (more of the same, but it's a good same).  EQ2 just came out and I've been playing that a lot with folks at work -- I'm "Hawksdottir" on the Crushbone server if any the 4 other people who read this are playing EQ2.  WoW is right around the corner and already the open beta is giving me a chance to play with some of my pals who didn't get into the closed beta.  I haven't even picked up Halo 2 yet -- you KNOW it's a crazy year when I can't play Halo 2 the instant it comes out.