Lots of discussion on this in the threads but the highlights are:
PROs- Off the ground, comfortable sleeping b/c of the mattresses, ease of set up/ tear down, features like LED lighting built in, companion annex/tents (for most clamshell style tents), very roomy, durable, saves space inside cargo bay due to being roof mounted. There are probably more, but you get the idea.
Cons- pricey (average over $1000-$5000), must pack up prior to moving your vehicle (unless mounted on a trailer), requires additional expense or modifications of a roof rack to mount it on top of a vehicle or trailer. Again there may be more but ‘‘tis is a start.