My RTT came with my truck so this'll be a somewhat unbiased review.
Are they easy to set up and tear down so you can drive daily? A la 100s in the hills?
Yes, if you're in good shape and not super short. Takes me probably 15 mins to set up or tear down, including the extra time I spend struggling with my old and janky zipper around the cover. Involves some standing on tires, in door jambs and hanging off the roof rack for me at 5' 9" occasionally, but it's not bad at all. Pro is you can leave your sleeping bag/gear in the tent. That saves a bunch of time.
Does not being perfectly level make it annoying to sleep on a slant?
You can generally find a way to park it to get it level. I've used rocks in the past, but with the addition of my 4 traction boards this hasn't been an issue. I'd say the RTT is vastly superior to ground tents in this area. Can't level the ground under a tent very easily.
Is the ladder a pain for getting in/out?
Could be. I'm 26 and in average shape so it's not a problem unless I'm super drunk. And even then I haven't had issues if I take it slow.
Is it too short to be worthwhile?
Not quite sure what you're asking here, but mine is the width of the roof stowed, but folds out to double it's width. I can comfortably sleep 2 grown men with tons of extra room.
Is it the kind of thing that is "cool" but not really worth it?
Depends on the price. If you can pick one up for a good deal, and will use it often enough, it is totally worth it.
Other nuggets:
The height is awesome. I've taken it to the infield at Talladega and it's glorious because people can't drunkenly walk/fall into your tent - offroad this translates to fewer bugs/animals. Also, gave me a front row view of the race with a window open. In the summer, it gets you off the ground and cooler, it's suprisingly more windy that high (might backfire in the winter, wouldn't know yet).
If I were to buy another one, I'd get hardshell. The PO of my rig didn't take care of the vinyl cover. It had some holes that I've tried to repair. It did fine until the summer, when I realized i hadn't thought about the melting temperatures of the materials I had used. Flex seal/tape is a no go. Melted and slightly gummed up the zippers like I mentioned. (It did waterproof it though, sooo...).
Mileage takes a hit - for me it's about 10-20%; even more so if you're speeding. I was caravaning with some bros in newer trucks doing 80-90 once and I was getting like 8 MPG with my 4 speed transmission... For that reason and to keep it from wearing excessively, I remove it in the off season - quite the task, but worth it. My RTT is NOT light. Which brings up the other major downside. It really lifts you CoG, and whether or not it makes a real difference, it definitely impacts my confidence while offorading sloped sections.
Finally, even on trips like the one from the pics below you can see how loaded up my cargo area is. It's nice to be able to carry more since you don't have to sleep in the rig. Allows me to stay out for longer without resupply and keeps me from putting stuff outside the truck when sleeping.
TL;DR: Nothing is perfect. But if you have the money and will use it, get one. They're amazing.
Lemme know if you hace any other questions! Pics for proof.