I have had one of these for about 8 months now, of all my bags it is the one I find I select the most.
I like the fact I can swing it round and get at my gear without unhitching a backpack and putting it on the ground to get at it. The amount of lenses I can carry amazes me, of course it gets heavier
.
Usual kit is gripped 5DII with 70-200 2.8 fitted plus 16-35 & 24-105. plus filters, batteries etc. (also a flash at the bottom of the bag)
The outer compartment is large enough but a bit thin and cannot carry anything bulky. The stretch side pockets can hold a water carrier and my hiking jacket straps over the top.
Monopod fits in the holder nicely, have also carried a CF 190PRO tripod there.
Although not a feature that is important for my use, walking the hills, it does provide a lot of security for your kit when wearing it as the pack has to be swung off your back to open it.
The AW cover also provides rain protection. All in all I think it is a fantastic bag.
One of the reasons I bought it was it had a decent hip belt so if you adjust it right the weight is carried mostly on your hips, that is an important feature for me.
Quite a good review can be found here comparing the 300 to the 400. The 300 does not have an AW cover so not a big decision for me that one.
http://photography-on-the.net/forum/showthread.php?t=553559
Bags are a personal thing but I don't think you would be disappointed if you bought one.
I hope this helps with your decision,
Paul.
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