I went last year. We had gold tickets over looking Eau Rouge but it wasn't cheap, and frankly not worth it. We spent some of Friday and Saturday watching practice form GA areas and there was plenty of great spots. It is very crowded around the Kemmel Straight, Les Combes and the turn as the car drop from Rivage down to Pouhon (this looked like the best spot), so if you wanted these places you would have to get there early.
If you're not in a rush you can still get some good spots with screen at Pouhon, this would be my recommendation. Take a fold out chair, lots of really good wet weather gear, c cooler box full of food and drink, and maybe a smal tarp and rope set up. A lot of people would sit at the fence, tie the tarp to the fence, and use it as a makeshift roof to sit under. The weather changes constantly and it bucketed down on the Saturday practice, this was the only time I was thankful for having a grandstand seat.
We camped at a really good camping ground and it was CHEAP. You can camp at the circuit, but after speaking to people who had done that we were glad we had not. It seems the noise at night does not stop there. Instead we camped down the road from Stavelot at this place.
It had very good facilities, a bar, good crowd, and there were tons of shuttle vans with no queue. When there were no vans after the race the walk back was only 30 mins. You can see on this map
the location, marked with a green arrow. If you scroll north you can see where the circuit is (the road the camping ground is off is the original Spa circuit). It was easy to get in and out, we arrived Friday morning and had no problem getting a good camping spot, and there is a supermarket a 2 min drive down the road, plus all the cash machines and town facilities in Stavelot 5 mins away.
Any other questions just ask.