Yeah, it is overrated. I find it quite a hollow experience, actually because, even though there's a massive landscape to explore, there's bugger all to do. It creates the impression that there's a lot to do but all you need do is scratch the surface and you find out that, actually, there's very little interactivity to be had. It's quite dark to look at, as well; 'Vice City' in all it's gaudiness and Florida sunshine was very bright and colourful to behold, but I find 'San Andreas' too dark (quite depressingly so, as it happens to be.)
There were too many rumours of things that never appeared in the game (some by fantasist journalists), and after you got over the shock and excitement of playing the finished game the overall effect of the game wore off pretty quickly. There's some beautifully done cut scenes, and a few cool missions (not including those bloody pilot training missions (Goddammit!!!)), and the main storyline was quite fresh in it's darkness and in being more emotional than usual, but after you've finished with that you just get the feeling that there's very little reason to continue playing it.
Having the character be black was a bold move and worked very well (I don't think you see that many black lead characters in games), but the setting of 90's L.A. gangsta culture inhibited the sense of fun in the game and they obviously couldn't find as much to take the piss out of, as they did for Vice. Plus, they weren't as over-the-top with sexual innuendos as they usually are, so that spoiled the fun, a bit.
Don't get me wrong, it's still a cool game and had many new, cool features in it, such as the eating, clothes, arcade games (fantastic, and an indication of the innovative size of the game), the ability to mod cars, ability to take pictures (one of the most important new features of ANY GTA game) etc, but the end result was just....missing something. In the end, I don't think they had the memory or the time to include all the features they could've done (and that there was said to have been), and as a result got too overambitious with size and thought that a relative sprinkling of hands-on gameplay would satiate players when, in actual fact, it just wasn't enough and the game ultimate felt like being a bit of a let down, and that it wasn't nearly half as good as it could've been.