I have been very happy with my Trifecta by Extang. It has been on my 2008 450 for 5 years now and have not had any issues with it. It is a tri-fold unit and if you have an 8' bed it works great to leave the hitch area open when folded up, but my coolers, gas cans, etc still are under the folded up cover.
There are a lot out there...I had a roll up with "rails" on my 2005 Silverado 2500 and twice I caught and damaged the rails on a sharp uneven turn with a 5th wheel...but...the 2500 was a short bed also so maybe that was part of it? The Trifecta doesn't use rails, can be put on and removed in less that 5 minutes without tools and has (at least when I bought mine) a lifetime warranty for the original purchaser...I like it...and will probably buy another one someday when I change trucks.
http://www.extang.com/docs/trifecta.html