The OEMs are clamps to the rail with theft deterrent screws and have a locked on cover. A determined thief could defeat them, but it would take perhaps 10 minutes for him to unscrew the clamps one he spent a minute pulling or breaking the lock.
There is a bit of fiddling and some tips that speed up installation of the cross rails. Takes maybe 15-20 minutes to install if you take your time, maybe 10-15 to remove.
Current: 2011 Grand Cherokee Overland V8, 2009 Liberty Rocky Mt V6
Previous: 2000 Grand Cherokee Laredo I6, 1979 CJ7 I6 Quadratrac