Don't have much experience with others pouches as I'm a newbie. Currently using a Digital Holster 20, Skin 50 and a Skin Strobe with a Pro Speed Belt.
I use this system mainly when shooting street photography in Singapore. Don't really like sling bag and stuff like that. I think it's great for wedding photography as well. The Digital Holster is great for changing lens. Just unmount your lens, stuff it in the holster, pull another one out from the Skin 50 and mount it. My Digital Holster 20 will fit a 70-200 2.8 IS USM, unmounted or otherwise.
Overseas, I use the Shape Shifter backpack with the Pro Speed Belt and Skin 50 without the Digital Holster. I have a 18-200 so I won't be changing lens as much.
I guess the main advantages of Think Tank modular system are the option of "purchase as you need" and the ergonomic design. The undesirable aspect of it is probably the price, though I'm sure that there are even pricer options out there.
There are probably tons of review out on the web which you can browse. I read through most of it before deciding on buying the pouches.