A comparison of rooftop tents and their pluses and minuses, from a guy who's owned them
First off, don't listen to the recommendations about rooftop tents from anyone who hasn't slept in one for at least a month. I have, so I feel adequately qualified to give you the scoop on the great (and not so great) things about this camping innovation. A few years back, after seeing some along the California coast, my wife and I became intrigued by the idea. We started looking online at a few different options, and fell in love with the E-Camper--- an aftermarket roof tent installed permanently onto the top of the Honda Element (maker Ursa Minor has since expanded to outfit the Jeep Rubicon, as well). Seeing some amazing images of people parked by the beach, looking out at the ocean while relaxing on a bed on top of their suv; the idea that we could just pull up to a spot, pop the top, and be ready to camp... this was a concept that just about any camper could obsess over. We watched all the video demos: how to set it up or collapse it in under two minutes, how to climb into it from the moonroof, and the features you could get, like reading lights and a power charger. The fact that it was built into the car not only gave it a cool factor, but seemed more convenient for spur of the moment use. We fell in love with the prospect of upgrading our camping experience, and that was one of the driving factors for choosing the Element as our next car.
If you've researched rooftop tents, you probably know that Ursa Minor was one of the biggest pioneers. The idea was an update of those old Westfalia vans, but integrated with a vehicle that is reliable and comes in a 4x4. And because you have to have it custom installed, it is so much integrated into the vehicle that people actually stopped me and asked me if this was a new model they could get at their Honda dealership. Unfortunately, this is not the case: I had to drive down past San Diego to the company's location, drop off the car, take the Greyhound back to LA, and pick it up a week later (the alternative was missing work and staying in a hotel for the week). Once it was on, it looked great. It added only about four inches to the height of the vehicle, and didn't effect fuel economy at all. You can even opt to have roof racks drilled to the top of it, so you could still bring a kayak, surfboards, etc. on your trip.
If you don't happen to have a Rubicon or an Element, another option is to get a tent that mounts onto roof racks. That way, you can use just about any vehicle (I've even seen them put on top of cargo trailers), you can remove it (with some help) when you're not on a trip, and it's considerably cheaper: our Ecamper ran $6,000 while you can get a basic rack-mounted one from Bigfoot or Autohome USA for less than half that. You can only get into it from a ladder, and it won't have cabin lights or a charger that runs off of your car battery, and it's definitely not as cool as a custom one, but I think the benefits outweigh the drawbacks.
Another major consideration is size. Our roof size allowed the bed to measure a roomy 4'x7'. Plenty of space in concept, and if you're on your own, or on the petite side, it may not pose an issue for you. In my experience, being 6'1" with a 5'10" wife and a lap dog that insists on sleeping with you, I endured more than a few nights tossing and turning through intermittent slumber. And if you have an active bladder, climbing over someone and Parkour-ing off the top of an SUV while half asleep is something you just have to get used to (God help you if you go to bed drunk!). One positive regarding the close quarters: warmth was no problem on cold nights, and if it got too hot, the canvas zipped open to a mesh layer to allow for a bug-free breeze.
Perhaps the biggest drawback of a rooftop tent is that you cannot move your car while the tent is up. So if you want to go to town or drive to a trailhead or take your boat to the lake, you've got to remove your pillows and blankets, collapse the whole thing before you can go anywhere. Sure, it may only take five minutes, but you have to set it up again when you get back to camp. If you have more than a few outings during your trip, it can get pretty tedious (if we had gotten a more elaborate fold out model, I would have probably hated the thing after a single vacation). If you're a person who likes to go cross country, stopping off at a different campground every night along the way to your destination, you want to have the easiest set up and breakdown, then this is an awesome alternative and I totally recommend it. But if you're staying at one camp for a while and driving out to visit different spots, a conventional ground tent may end up being more convenient, and a heck of a lot cheaper. When I go camping, we usually stay for a week or two at one campground, taking time to explore the region, engage in different outdoor activities, and see what experiences a nearby town has to offer. Maybe we'll get to a cross-country trip one day, but for now, it turns out that standard tent camping works fine for us.