I got it when It was 40 and I had/have had a blast playing it. at 20 dollars it's a steal.
I say get it for now and seeing as though most retailers have some pretty decent mid summer sales (to combat the lack of games during that period) wait to see if there is A a price drop or B a good BOGO style sale for the summer months.
20 dollars and the general consensus is that most everyone loved it, but if you didn't like it you won't have shelled out 50 for it. It is a great game though.