@Cyberlawyer - The power variation was one area of concern when I owned a GBBR. The other issue which I was conscious of was the additional encumbrance of carrying a sufficient amount of gas versus just carrying an extra battery or two.
Admittedly I am heavily contemplating returning to the world of GBBR's. Despite using low caps wherever possible, I find I miss the reduced ammo, stop on empty and tactile feedback of a GBBR. The NGRS Scar-L I picked up from
@Davegolf answers a lot of the previous list but unfortunately my true love is the MP5 and the AK (74 and later) platform.
On an unrelated, what's the general opinion on the MP9 from ASG/KWA?
@Reginald - You've got a lot of good information in this thread and I would echo what several others have said when it comes to trying the hobby/sport first as a rental. I'd also strongly recommend obtaining your UKARA prior to making any purchases. I would argue that GBBR's are easier to maintain (based on my experience with the MWS) but the upkeep is more intensive in my opinion and more costly simply due to additional cleaning/lubrication products required. The MWS was a very easy platform (in my opinion) to maintain, unfortunately I can't comment on any other GBBR's.
I'd always argue to go with what you want as it's your money and only you know what it is you're really after. GBBR's definitely have a more significant outlay in terms of initial cost but depending on your choices when it comes to an AEG you can end up spending just as much, if not more.
It might be worth considering picking up a mid-range AEG once you've tried the hobby, confirmed you enjoy it and have obtained your UKARA. If you pick up an AEG in the same platform as the GBBR you're interested in then this may help confirm whether or not you like the feel of said platform. If you get a good mid-range AEG then it would also give you a solid back up once you pick up your GBBR.
Just a few thoughts.