In all seriousness - the thinktank bags are the best rolling camera bags that you can buy, and well worth the extra money over anything that lowepro can produce imo. There is a certain... completeness to them, every single detail, no matter how minor, being carefully thought out, every single zipper or pocket having genuine, logical decisions put into them.
Pay the extra cash - it hurts, parting with over £200 for a bag, yes, but it's completely worth it, I'm inseperable from mine. There's a whole host of reasons that so many photographers rave about the airport international - whereas I don't think I've seen any professional evangelising the lowepro rollers.
edit: if you're used to GEM £30 backpacks... nuf said that lowepro's professional line's quality would blow you away, thinktank will just make you throw out your GEM bags and buy more thinktank
edit: of course, mine gets rigorous use, very regularly, in all enviroments - i was dragging it through the snow at 5am for a job a few weeks back!. If you just do this for fun, then you might well find it easier to justify something a bit cheaper, but probably still 80% as good