This game is definitely the best in the series, R* take their GTA franchise to another level. The game controls are superb, most notably, the vehicle handling - it can take a while to get used to though but, when you do, you'll really appreciate the improvements. Great dynamic lighting and weather effects. Some amazing attention to detail and immersive atmosphere. Some elements you'll be accustomed to from previous games have been redesigned, while some don't feature at all. There is a grittier storyline than in previous titles, you won't find a 'rise to power' theme, you will find some great characters and it oozes more quality and polish than previous titles, even though they may have also been amazing at the time.
The addition of multi-player delivers several modes to appeal to all tastes. Great fun to play, it can be absolute chaos but, when you've got a city to roam, a huge variety of vehicles, weapons and the nature of the game, that's to be expected. You'll find 'lone wolf' games, team games, co-op games and a free mode where there is no specific goal.
If you're expecting this to be some kind of extension to GTA:SA then, you're probably going to be disappointed. There are very few side-missions, no customisation, no empire building, no garages, no unlockables to be stored at your safehouse, no planes and many other things from previous titles you just won't find in the game. So, if any of those things were integral to your enjoyment of previous GTA games then, it probably isn't for you and I'd suggest you do some independent research to check exactly what the game does feature.
However, if those weren't important features for you then, as long as you appreciate solid and robust gameplay, good game design, multi-player appeals to you and you're happy to have a learning curve (the vehicle handling and the multi-player mainly) then, this is a great choice for you.
With the DLC The Lost and Damned and the soon to arrive The Ballad of Gay Tony, it's a superb choice.