The magura louise is a good brake. I've never personally used one on the rear but have done on the front. I'd say there about 95% as powerful as a hope not as much feel though a touch wooden prehaps but they are a fair bit cheaper too so you get what you pay for. Several manufacturers have used them on their full builds so their not completely useless!
If you can live without that glass like infiniate modulation of the hope brakes the louise is very good, i used a braided hose and sintered pads which helped also. But its down to you mate.