I have both the Lowepro Slingshot 200 & 300. I like them alot. I tried out the Tamrac Velocity, but they didn't hold enough for me, and the build wasn't as nice.
I can put a dslr + 50-500mm in the Slingshot 200. I have a 20D + 300/2.8 + 1.4xtc + 2xtc + flash + 2nd body and batteries + 18-50/2.8 + a Bessa R w/ 35/2.5 in the 300. Heavy as heck, but it does hold it well, and I've gone bike-ridding with that too.
One thing I like about the Lowepro, is that I can shorten the strap and just sling it over one shoulder and the stiffness of the strap & padding helps it stay on the shoulder. Lengthen the stran, and you can carry it across your chest. The Slingshot will also swing around from the back to the front easily.
I mainly use the 300, and my wife uses the 200, but I might get another 200 for days when I don't feel like slugging the 300/2.8 around.