I've seen them, never really felt the need to buy one If its for riding in the rain I have a cheap slicker that looks like the drizabone that probably does a better job of keeping me dry. I think It was around $30. I like my even cheaper rain poncho best, it has a hood and I can spread it out like a tent over the saddle.
I have always just used the cheap yellow slickers...you can stuff them in a tack box or anywhere for that matter and I don't have to worry about loosing somthing that I don't use much or having it stolen when we are at camp. Although I wouldn't mind having a really nice one, guess i'm just cheap...
I've had mine for well over 10 years now and never re-waterproofed it, I guess if needed, you could use the same stuff that you can get for canvas rugs. I've never washed mine, it's never needed it, I don't know anyone else who has either.
Once broken in they are really comfy to ride in, when they are new they are a little stiff at first.