I have had great luck with my "Cover-It" brand garage.
I think its the 12X20X8, with the peaked roof style, not the round roof. I have had it up for.....geez at least 8 years now I think. Went up really easy. Hit it once with my car because of ice in the driveway, didnt phase it. Its at my dads, which is on the top of a hill, and we are in upstate NY, so it gets all sorts of weather thrown at it, snow ice sleet, winds, everything. And I only could get half the anchors in the ground because of our driveway, so considering that, Id say its well worth what I paid for it. The covering is great, although I would recommend some sort of water barrier for a floor (tarps, etc) if youre gonna store anything in it, since it retains moisture easily. The peaked roof is a lot easier if you plan to work on anything in there, you get more headroom than the rounded roof. I also opted for the roll-up door kit, which makes getting in and out easier.
Hope that helps! I think you can find the web address or a phone number in the back of any hot rod or car craft (which is where I found mine). Good luck! -Jason