I've got a Lowepro system. I looked at ThinkTank as well, but liked the robustness and relative weather resistance of the Lowepro stuff better, though the lens carriers are bigger due to the extra padding.
I've found it very useful when doing events or some sports where I'll typically take a backpack full of kit, check the venue, then pick out the individual items I want and load up the belt kit. Typically I might have a 300 and 70-200 on the cameras, and then a couple of shorter lenses, an extender or two, a flash or two, cleaning cloth and other sundry bits in the belt kit.
It's much less hassle to carry stuff that way compared to in a bag hung over your shoulder (or worse a backpack that you have to put down on the floor to get into), and all your bits are within easy reach.
The only problem I can think of is that it will make you rather "wide" when squeezing through crowds.
Recommended if you're out and about a lot doing events type stuff.