I use the higher model, the Lowepro Flipside 500 AW, and I absolutely love it.
Its very comfortable to carry, it has plenty space (well not unlimited but enough for any reasonable amount of gear), it allows to change gear quickly and comfortably (close the belt, move the back to the front, open it and you can use it as ideal support for quick lens changes), and it protects your gear very effectively against both bad weather or theft.
If I would be asked what could be improved I would ask to get rid of the stupid front superflous department and rather give some more space in the back. But oh well, for the moment I just keep that department empty.
Theres also a possiblity to carry a tripod, but I dont use tripods.
All in all, the only thing I really want is a second 500 AW so I have a spare if something happends to the first one.