I recently bought this cover to go over to outdoor wall mounted mountain bikes. The bikes are 21 and 19 inch (extra Large and Large).
I have not trouble getting this cover over the bikes, although because it is shaped it is not simply a matter of draping it over. The cover is elasticated at the bottom and has two elasticated hooks of the bungie variety at the bottom, which you can use to fasten it down. It stayed on perfectly in the recent gales, when a cheaper cover was mostly blown away.
I can't say how durable it will be, but it is way stronger than the sub-£10 covers I have previously bought which tend to tear the first time you use them.
For me I think this represents good value for money.