I would not line "GTA" up in that "series with little improvements"-line, though.
Or you could just as well line up the whole RTS- and FPS-genres as well.
As for the initial question: It always depends on it. There are GOOD "uodated" series and bad ones. There's definetely no way to generally say "games-serials are bad" or "good".
Look at Mortal Kombat for example. The first part was new and exciting, the second part enhanced that, but things like babalities also took away from the athmosphere and then every next installment didn't even get near the greatness of the original two parts, in fact things went downhill pretty fast from there. But now, in the official sixth installment (deception) - not to count all the spin-offs and "best ofs" in - you have a game that beats every other MK hands down. But the series wouldn't have gotten there without the lamer parts in between.
Same for GTA. I for one didn't care for the first two parts (and "London 1969"), GTA 3 on the other hand managed to really get me into its grip. And Vice City was even another giant step further. In terms of gameplay and ideas, San Andreas AGAIN took huge steps forward. The only reason I still like Vice City just as much is because I prefer the 80's-style "Scarface vs. Miami Vice"-look and feel of Vice City more than the "Hood"-feeling of SA.
There are loads of other examples for that, but of course also examples for games that sucked more and more with every installment (up to MK
A the MK-series was in fact a perfect example for THAT), so as I said: There's no way to have a "general" opinion on it...