I use the Lowepro Phototrekker too, and find it a very good backpack.
It will carry my 20D with 300mm f2.8, 70-200mm f2.8, 17-40mm f4, Contax RTSIII + 28-85mm and 21mm distagons, along with filters, film, batteries etc.
When fully loaded this is obviously very heavy, but it feels really comfortable because of the great design. Most of the weight is supported on your hips and it's all very well padded, and easy to adjust straps and shoulder supports to your individual build and proportions.
There are of course smaller models for those who don't want to be a pack-mule!