I asked the same question just on a different forum. These are the reactions I got.
Went there for the first time last year for the GP, found a few nice spots.
1) Pathway alongside Eau Rouge: There's a spot where you can shoot through the trees looking up the corner, Great for some naturey shots.
2) The left hander after Rivage. You shoot looking down right on top of the cars, its a brilliant spot and I got some great pics of the GP2 cars there.
3) Entry to Rivage: Wierdly there's some huge metal wheely bins (doesn't look like they move) that allow you to get shots over the fencing as the cars exit Malmedy and head down the hill. Bit sneaky but worked well!
4) Malmedy. On the inside of the circuit theres a little path in the trees that runs right by the fencing on the inside of the corner. Quite tough to get pics as you will spend most the time trying to keep your balance but definitely worth trying. Access is via a small pathway to the left of the pedestrian tunnel under the straight between Malmedy and Rivage.
5) Bus Stop: the new layout means you can get great shots as the cars brake into the corner and turn right. If you are heading away from the pit straight, walk past the grandstand below you until you reach a (probably closed) waffle hut. From there there ought to be a section with camera holes in the fencing. You will get awesome shots from there.
6) Blanchimont: The public pathway runs slightly above and close to the track here. If you can climb up the hill a little it and shoot down over the fencing you should get great shots of the cars through the trees. Loos great if your camera is up to the task!
he huge grandstand opposite the main pits offers potential - especially if it is open, free and it's raining! Also the roof of the pits complex building - worth investigating if you have some reasonable flexibility in your lens options. If not it's still good for overheads into the pit lane (I assume the event will be using the main pits.) and the upper paddock area.
Either side of the Eau Rouge pedestrian tunnel offers some impressive images for race starts as the cars stream down the hill.
If you have a long enough lens available the steep path up the outside of the circuit at Raidillion has some places for looking down into the Eau Rouge section.
Kemel straight is a bit marginal but in some places you can get fence free shots of cars that are keeping a bit right on the track. Les Combes has some possibility when passing through but I would not see it as a prime location - maybe with a step ladder and a longish lens ... or a (mobile) rubbish bin as I saw some people braving?
The section past Pouhon through to Stavelot seemed to have too many fences and no shoot throughs that I could see (last September) so probably best to avoid if snaps are you priority. If pics are secondary the racing should be interesting there but there are a lot of fences.
I totally agree with the previous recommendation of the left hander after Rivage and then all the way down the hill to Pouhon if you are into panning shots from above the cars.
Allow plenty of time if you are planning to 'learn' the circuit. It's big, the hills can be steep (especially at Raidillion) and there is much to see with great surrounding views everywhere. Fitness helps .....
One thing to watch out for. The main agte near Stavelot has a Kart track alongside it and the circuit fence is very close to that with a pathway. However, photographically it's not a great place to try to do much and the path has a fance (at least it did last September) that means you can;t walk all the way along the side of the track at that point. In fact there is a fence at the 'back' of the kart track as well. With some help I found a way to get over it last year but failing that you would have to re-trace your steps. It may be better to cut that corner completely and take the path that heads through a tunnel under the section between Stavelot and Blanchiment and puts you on the outside of the circuit.
Of course if you have the time and energy the path beside the kart track is interestingly close to a high speed part of the main track on the other side of the fence ...