These stickers are very good quality, both in terms of the material, the colouring and the pictures themselves.
The one thing you should bear in mind is that they are very large - we covered a whole room from floor to ceiling using one pack of these, and to be honest if you have some furniture against the walls (e.g. a bed) then there are probably too many stickers.
You also need to be very very careful when taking them off the sheets to put them up (as they are still attached to their surrounds slightly and could be torn easily) and also when taking them down to move them (as they have a tendency to curl back and stick to themselves). Beware that if they do stick to themselves they will be ruined as the adhesive is very strong when attaching to itself (there are no problems on the walls though).
Many people have mentioned that they 'peel' off the walls a little, and I have found that the edges do start to raise a little, but all this means is that if you see this you run your hand over the sticker and everything is sorted out - this doesn't happen often, ours have been up for about a year and we have had no real issues.
If you are considering such stickers you should also look at RoomMates stickers (which Amazon also sells), which are a little cheaper and contain slightly less stickers (probably the right amount for a medium child's room that contains furniture).
UPDATE: 10 MONTHS ON
We've had these stickers up in our child's play room for 10 months now. Unfortunately they are beginning to curl at the edges and come away from the wall, and try as I might I cannot get them to stick back on properly (irrespective of how much pressure I apply).
I had previously given the product 4*, but I'm downgrading it to 2* as I think a 1 year product life is a bit poor (plus the idea that you are going to be able to move and re-stick these as time goes by is obviously ludicrous)