My opinion: I got it for the PS3, and with that controller, I loved it. Bought the DLC, spent a lot more, and got great value, probably played it over 100 hours.
I don't like racing games usually, it's the only one I have and I thought it was a great balance of racing, exploring, variety in environments - city, mountain, shore, etc.
It has some reward for getting to know the city a lot, over 100 'secret rewards' to find, some of which take a good while, a 'combat race' mode.
The music does get old and repetitive, but not that big an issue. The theme song fits it.
The one mode I didn't like was the 'stunt' event, and the game doesn't require it.
So I think it's a really great game - but the caveat would be that I don't think I'd have enjoyed it nearly as much with keyboard/mouse.
In fact, I bought it on PC after that and never even played it. I also played it on a big screen tv, that added to it.
So, all kinds of carrots and variety to keep you playing - tired of racing go look for another hidden spot, switch cars, go to the island on a motorcycle, jump from buildings.
Hope that helps - great game but perhaps mostly with a controller.