If you don't, you are missing out like you will never understand. It is very important that you buy this game to experience the best game ever made. I dont care what anyone says. I will never stop playing this game, and there is still so much to discover.
It's worth it. Honestly, it felt like more of the same at first, but as you play it you'll notice the slight differences, and they're all good... if you liked GTA3, this'll keep you busy for a long time.