I haven’t used the Roll N Lock, but I’ve written a bit about my Retrax Pro XR, and I like it a lot. XR adds t-slot rails, which you can use to add racks (Thule, Yakima, Front Runner, and probably others), bike fork mounts, and other accessories.
Benefits of retractable covers are convenience, security (particular aluminum versions), and reasonable waterproofness (don’t expect them to be completely waterproof, but I’ve taken cross country trips through heavy rain with little water making it in). They also don’t block your rear window like folding hard covers.
One feature that I like about Retrax is that they can close and lock in any position; some will only stop every 6 or 12 inches. Retrax is also nice that the “handle” to open and close it (which is also the locking mechanism) is on the side, versus a cord or tether on some models. This is a minor improvement, but nice to have.
Biggest drawbacks are price and losing about 10-12 inches at the front of the bed due to the canister where the cover rolls up. And they aren’t easily removable if you need every inch of your bed.