If you're looking for something that does it all -- moderate hiking, travel backpacking, converts to a duffle for slightly more professional travel, is waterproof and amazingly well designed and lasts forever, check out the REI Grand Tour Line.
I have a 2000-2002 series of the Grand Tour, the women's version. For my money, it is the single best piece of luggage ever made by the hand of man or god. The first one I bought in 2000 got peed on by angry cats and couldn't be cleaned, and I haunted eBay for 10 YEARS looking for a replacement, because I loved it just that much.
The current ones are pretty darn okay, but that's only in comparison to the GodBag. ;)