In reply to Silverdene:
I have one of these
http://www.thenorthface.com/opencms/opencms/tnf/gear.jsp?site=NA&model=... (the XL) and it is excellent. I have travelled all over the world with it and the longest I have carried it for was about 5-6 hours and it was comfy. It compresses well, the straps tie up well, if you don't fill it, it doesn't feel terrible, it is really good for living out of and locks. I've had mine for five years and the strap is starting to go on the right shoulder (I am right handed, so naturally sling it there for short distances.) I use my left shoulder now (cunning, eh!) and hope to get another five years out of it. If I was to replace it, I would look at other models and go for a similar size, robust (gets a lot of abuse and 140L is really heavy when it is all on one strap, I had 44 Kg in it once.) shoulder straps, compression straps.
I normally go for cheap stuff, but this is one area where I would get the toughest I could find, as having a broken buckle/strap/leaking bag in the middle of an airport, on top of a bus etc... would be rubbish!