I've been diving with a DUI W&T since OW Checkouts -- borrowed a W&T II for checkouts, then bought a W&T Classic (the one that's webbing based) right away. Despite being on the large side, I don't really have much to hold up a normal belt - no butt or hips. For checkouts we were really overweighted (and diving 7mm john/jackets, in xxxl) and it was no problem carrying a bunch of weight (at least not from a carrying perspective -- diving overweighted sucked!)
The Weight And Trim works beautifully for me. I can place the weight as high or low as I choose, and buckle placement in relation to the BP/W is a non-issue since it's not used for ditching weights. The pockets on the Classic at least are individual flaps which makes it possible to add/remove contents without as much chance of losing necessary weights. On the surface, It's comfortable enough where I tend to forget sometimes to take it off after a dive.
I don't think the string configuration for the weight pockets is an issue. Only takes a couple minutes to re-thread. No, I couldn't do it while wearing the harness, but I don't expect to ever have to do this.
My wife has the DUI Delta Wings BC which uses the same pocket system. Works pretty well there too, just integrated into the BC. With the velcro pockets, she can don the DC and then put weight in the pockets after she's got everything adjusted.