I'm with most here who enjoyed GTA III; Vice City; and San Andreas (my favorite of the series thus far), but don't love IV so much. I'm actually playing through IV now (as I've never beaten it before), and while I am enjoying it for the most part, I feel.. trapped. Trapped by being in 1 unimaginatively designed city. Trapped by having to go on dates and friend dates to get some game play benefits. Some of the missions feel stale and repetitive as well... Don't get me wrong, I like the game. I just don't enjoy it nearly as much as the 2 main releases preceding it...
That being said, I'm looking forward to this new one. I initially didn't like the idea of having to keep up with 3 different protagonists in a game like this, but I suppose it'll add some new and interesting tweaks to game play. I'm happy that there will be several types of vehicles to pilot, and that the environments are going to be much more varied and expansive. I can get real lost in that alone. From some preliminary articles I've read, the missions are apparently going to be much more interesting and exciting, as well.
Time will tell, natch.