Apart from V, I've only ever owned IV myself, but as everyone and their mother who had a PS2 seemed to have San Andreas as well, there were lots of opportunities to try that as well, but I don't see that as much of a loss, since GTA has always really been about the free roaming to me, so it wasn't like I constantly had to try the same missions again and stuff :p
Only reason I actually bought IV was because my friend and I were looking for a game we could play together, and as IV was on sale, we decided to buy two copies of it (well, we decided that I bought the games, and he'd repay me later. The money is still nowhere in sight, and this was like 2 years ago ;p), and it turned out to be a lot of fun. I was a bit bummed by the lack of a flying cars cheat, as flying around the world in a car was one of the things I had the most fun with in SA. But anyways, all my time in IV has pretty much been spent on the online mode in IV, as the story just never meant anything to me in the games, and I really don't feel like I've missed out anyways. Doesn't sound like it either, with all the criticism the story/missions have received anyways.
It took a bit before finalizing my decision on buying GTA V though, as it was quiite expensive, and without a job, I'm kinda cautious with what I spend my money on, and as I also had a lot of other things I wanted, I wasn't sure if V was gonna be the first on my list. However, about a month before release, Gamestop started a deal, where you could give them two games from a list they had, and pre-order the game for $20, and it worked perfectly, as I had two from the list which had turned out to be huge disappointments anyways ;)
But yeah, so far the game's been great. The story mode of V is certainly more interesting that IV, so I don't mind being stuck with it for now, but the online mode is still what I mainly got the game for. And from the online trailers, it looks like it's gonna be awesome this time around, especially with how big the map is in this game :D Still no flying cars though :(