I just bought a Lowepro Photo Trekker AW for my 4x5 monorail and it works very nicely (though a bit on the heavy side with all that gear!). I've never tried a Tamrac bag out, I've always liked Lowepro. I've used the Orion Trekker, Orion Trekker AW (a very different and much larger bag), Rover AW II (I loved this pack! It just wasn't large enough for my 4x5 equipment...). I also use a LoweAlpine (same company) pack for "backpacking" (I've actually yet to go backpacking... so far it's only been used for overnights and camping).

On a side note... I think Lowepro backpacks really only shout out "I'm a photographer!" to other photographers. I doubt many theifs, unless they're photographers or have done a lot of research (which is possible, but they might as well just get a job instead!) really know what a photo backpack looks like. That's just my opinion, but I have yet to be held up at gunpoint or anything... On a side note, I'm willing to bet my Photo Trekker with all that equipment is pretty darn bulletproof, so I could probably run away and be ok! Until I got a heart attack that is...