My old AE1 was leaking like a sieve - I picked up the first kit I saw - it was from Cameraseals.com. First off, the work is simple, very simple - the only thing required is some patience when removing the old material (by far the hardest/most tedious part of the whole process). Anyone can do it.
Last but not least - I have since seen John's (interslice) kits... well, not that the one I got was bad, but lets put it this way: after seeing the quality of the foam, the kit, the instructions - I know where my next order is coming from! Having tried the competition, and even though I did not have a bad experience, I would have to add my vote to the ones already cast for the interslice kits.