The Spa 24 Hours is an excellent race, very different from Le Mans though, despite similar cars being involved.
It's not as extravagant as Le Mans can get in terms of the drinking and partying etc. but we still manage to have a good time!
I would reccomend the Eau Rouge campsite myself, particularly if you are going for a few days, which I also reccomend! It has decent facilities, showers which are a godsend, and very friendly. In contrast, the Les Combes one has nothing like that and is fairly inhospitable. We stayed there in 2005, luckily only for one night. I wouldn't want to spend a week there!
In 2006 we moved there on the Saturday morning, as we weren't planning to sleep much, so didn't want to pay for another night. We never went back there until about 6pm on the Sunday, when it was very bleak and empty, and chucking it down with rain. Not ideal for trying to take down a tent!
This year I think I'll be in a hotel though...
I would reccomend the bar behind the pit buildings (if it's still there!) on Saturday night as well. Also, visit it on every other night, morning and afternoon. You know it makes sense!