Sorry I missed the question earlier.... some responses:
1) humid conditions our Maggiolina was at least as pleasant as any 'normal' tent would have been. In fact, being up higher gains you more breeze and gets you off of the hot, damp ground. Major plus. Also, when you are right on top of the truck, it is easy to add a Hella turbo fan to the tent. Draw is less than 200ma (I think?) but the amount of air movement is enough to draw sincere envy from anyone camped nearby without a fan.
2) I wouldn't put more than 4 boards on the tent. I've had two 9.0's, a 7.7 and a 6.0 on our tent with no problems. Harder to lash them down well as there is some flex to the tent top... but not a problem. The shell is glassed just like a surfer, so neither one seems to really dent the other.
3) No heavy stuff on top of the tent. Bad idea. We had room behind our tent (on a 60) to fit a couple of jerry cans. It wasn't ideal, but they fit just fine. Or a toolbox, or something else of about that size (the Balum.com folk had their ladder mounted on two Thule bars..... could have been in some way related to a Thule sponsorship).
4) Camping on the truck isn't always perfect.... but you can also get a bivy sack if you really really need to be able to get some distance. Or another tent. We carried an extra tent for an entire year for just that reason. Never used it. Not once. Welll, ok, we did use it-- loaned it to someone else for a few days. While imperfect, camping on the truck almost always works just fine for the camping we did. dd113 mentions the mudhole-- to that I say: you'll never want a rooftop tent more than when you've just spent a few hours trying to repair your broken truck (in a mudhole) and finally give up... and all you want to do is crawl into a dry bed. But it is raining, and you are filthy. Pop the rooftop tent, climb up, towel off, crawl into a BED instead of a damp sleeping pad in a damp tent..... it is fantastic. Inclement weather makes you giggle over how happy you are to have a rooftop tent instead of a muddy, moist ground tent. I promise.
5) Level. We carried two redwood 2x6 planks that were about 6 inches long. Most of the time we could get level enough with some manuevering and the couple of inches of lift under one or two tires. Sometimes we couldn't get level. But sometimes you can't find a level tent site either. That's just the way the cookie crumbles. We all carry shovels too. Digging down two or three inches for the high tires works wonders to level out the car, but it cuts into the Corona time much more than a couple chunks of wood or some rocks. Coconuts don't work. Even if they survive the initial driving onto them, they might explode in the middle of the night and wake you up with a sudden "Is someone messin with my car?" panic attack.
In summary: I couldn't possibly endorse a rooftop tent more stongly than I have. For infrequent weekend camping it is probably overkill. I agree. If you camp frequently, or if you are on any sort of expedition where you travel, camp, travel, camp... a rooftop tent is FANTASTIC and well worth the expense. Oddly- I would say that on a smaller vehicle (40, or passenger car) a rooftop tent is even better as there is no option to sleep inside, which you CAN do on a 60, 80, or 100 series.
Whew. That's about all I have to say on that one.

Here's a gratuitos photo of the tent on our fj60 and another on the fj40.
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91 fj80 w/ 93 1HD-T. OME 850/860 w/ 1" front spacer, LEDs, HIRs, Hankook MT's 31x10.50r15, Tuffy center, ARB rear locker (in progress), in garage: lightforce lights, full rack, superwinch X9, maggiolina tent, hella horns. WTB: Tan DS mirror & housing
'88 HJ60, "Louie" For Sale in Costa Rica.