This year's
Bristol International Balloon Fiesta is 11th - 14th August 2011. If you find a decent spot, it's usually quite an impressive sight, providing it doesn't rain - a dry weekend is by no means guaranteed, especially in August! Not sure if your trip will overlap this weekend at all?
As for the uniqueness of Bristol, on the surface it's just another English city in many ways. A bit more laid back perhaps, quite a wealth of creative talent (certainly in terms of animation), and there was definitely a thriving alternative scene when I grew up. But don't expect some magical ambience to hit you! If you hunt out the birthplace of Tricky and end up in Knowle West, you'll know what I mean! (Ditto Portishead - great music, shame about the place!)