I've managed for years with the Lowepro Orion Trekker but had realistically out grown it, especially when I upgraded to the EF 100-400 lens. It would only fit in the unprotected top section.
With the Fastpack 350 I can comfortably fit the aforementioned attached to my 20D with my standard zoom, 60mm macro, flash gun and a portable hard drive with room to spare. Add to that the front pouch and the day pack section, which comes complete with cable pouch, and I find I've got more than ample storage for batteries & chargers, filters, spare cards, cleaning gear..... OK you get the idea.
The carrying system is yet another strength of this bag. I've not put it to a long day trek test but around town it's very comfy. The straps are well designed, with the sternum strap much better thought out. The ventilation system not the best but adequate and the hip belt sits comfortably for me. The main strength here is the ability to swing the pack to the front and get to your pride & joy without taking the whole rig off. Not quite as easy as the sling type packs but a good compromise for those who need the full harness for long days out.
Is there a down side? well yes. This pack is definitely a compromise albeit a good one. The main thing that lets it down, however is the lack of a tripod carrier. That could be a problem if you intend to go out for a long trek looking to do some serious photography.