I've seen a lot, worked with a few, but never actually had one on my truck. I personally like the kind that you can get the hell out of the way if you want to. The hard covers that just raise up with hydraulic arms a little bit are pointless to me. To put in a bit load you have to go through the ordeal of removing and storing a giant piece of fiberglass. The kind that roll up or retract out of the way give you security, but allow your truck to be a truck too. I like the vinyl kind that have structure built into them. Kind of like this...
http://www.jcwhitney.com/wcsstore/jc...0368G_CL_3.jpg
Those are cool because they get out of the way quickly, but still offer a measure of security. My father-in-law has one like this and if you have the tailgate closed and locked you can't get in because you have to trigger a little release on the cover from the inside. It's only vinyl though so still susceptible to someone who doesn't mind slicing their way in.
That's where the hard retractable ones come in. I think you get the best of both worlds with something like this...
http://www.jcwhitney.com/wcsstore/jc...4686G_CL_1.jpg