My Isofix seats (second stage - 9m-4y) do recline, and both DS's have slept comfortably in them. I wouldn't move the seat with a sleeping child in it - it would be very heavy and awkward. This seat is particularly heavy, but I probably wouldn't do this with any second stage seat. Instead, I tend to transfer to a stroller to move a sleeping child.
Most airlines don't weigh seats as part of your baggage, but I wouldn't check mine into the hold personally (would worry about invisible damage). It would be a faff to carry on though as it is particularly heavy (in relation to non-Isofix seats).
Agree with MM re pro's - even seems sturdy when you've forgotten to use the seatbelt to strap it in and have just used the isofix points. (DH ).
My DS2 can also escape the straps. But perhaps he could do this with any seat. . Agree that it is hard to clean, but with time I've got used to the way it all disassembles (we've had two, so I've had plenty of experience cleaning them).
We bought one for DS2 as DS1 was still in his when DS2 needed one, and although he was ready for a booster I preferred to keep him in the car seat up to the upper weight limit. Check the deals around. We got our second one cheaply from Kiddicare, and sold the 1st one for more than the new price of the second one on Ebay.
Check when you buy your new car that the Isofix points are easily compatible with the seat. Certain models of car are easier than others. We had to have our seats cut to use the Isofix points (Renault Laguna Sports Tourer). I've loved the easy transferability between our car, friends cars, relatives cars... Would definitely buy one again.