I use bike covers in my condo garage and they work great of keeping the grunge off. I also used them on my balcony at one time. I tried inexpensive and they just ripped, mid level which lasted maybe half a season and great quality which have lasted almost four years. Unfortunately I can't give you a recommendation of brand name as I purchased them in Japan.

One particular nasty winter here in Toronto, we had a week of constant freezing rain and my lock would freeze up and the bike would be all icy I brought a cover to keep the water and ice off. It was in a locked bicycle cage at work so I was able to keep the cover overnight there without worries of theft.