Theoretically San Andreas was a great game. Except for the (at least i felt so) lack of style that Vice City had (i guess its an unfair comparison), it had a much grander scale and was in terms of Gameplay pretty much better and/or at least as good as the games before (especially on consoles, where you could finally aim manually)
However, by the time San Andreas came out, i was pretty tired of the same 'ol GTA. The new 90ies Gang Setting was interesting enough, but after you first got out of the Lost Angeles-ish City, you did the same total over the top stuff as in other GTA's...worse even, given the mission where you sneak into a hidden military base.
Of course there were lots of things to do...but as it turned out, not many things really were this interesting (oh god, i just remember the horrible truck driver minigame).
And of course, the graphics were really aged at that time. While it wasn't that big of an issue, it really made one of the greatest new things, the countryside between the citys, look either boring or horrible.
So in the end, GTA: San Andreas was a very good game, but since every good thing here came with another flaw there, it felt like "just another GTA Game" in the end...and it was the third one...and the 90ies Gangsta Setting was really a hit and miss thing. So it wasn't really loved by many people.
Also, when i took my virtual GTA girlfriend for my first date, she wanted to do a drive-by....WTF?