Hi,
I have owned a couple of roof tents and used them in different terrains. My first rig was a autoacmp 2 berth canvas roof tent on a 300tdi disco. I then moved onto a hanibal roof rack frm ebay and a quetcha 2 second tent. My current rig is an autohome columbus variant on a TD5 90 from BOAB.biz. (See my pics)
Pro's & con's:
The canvas fold out roof tents have lots of room and are nice and airey in hot climates, however when wet they stay wet for a while and they are noisey in windy weather so not ideal in Europe. I used one in the alps for a month with rain every other day. It held up but was not comfortable.
The roof rack and quetcha option is cost effective and does the job but wont stand up to expedition use especialy if you are tight for space and need to use the roof rack.
The Autohome columbus is very quick to errect and seems to be comfortable in extreme weather (North Wales (Cold&wet), Spain (Hot&Dry)) I have had it on the roof for 1 year now and it has no noticable effect on the fuel economy, it also gets used by the wife and I at random times like a new years eve party or when a lot of relatives come to visit (Very handy) - The downside is that they are expensive.
In a nut shell go for the best you can afford, pitch it wisely. If you are off to africa go for one designed for african use like hanibal or easy-awn. If you are staying in Europe then the same applies, go for one designed for Europe climate.
Re: Explorer/Overland Roof Tents.
There is a forgotten, nay, almost forbidden word, which means more to me than any other. That word is ENGLAND. 
