I am 5'9" and weigh 155. My body fat is ~15% (using an admittedly inaccurate device). I am using the belt to train while trying to get over a one-year battle with shin splints. I have now used it 12 times over 16 days. I read the recommendations and decided the Pro was the correct belt. As it turns out, I could use a slightly more buoyant belt and probably would have been better off with the Pro Plus. I'm not a muscular guy, so I must be one of the "sinkers" they were talking about. I find that I need to constantly expend some energy to keep my mouth above water.
The fit for me is tricky. If I push it low enough to get my head out of water, it is uncomfortable on my back. I also have to get it pretty tight to keep it from riding up, so it digs into my lower ribs and makes breathing a bit more work. I have countered this by engaging my abdominals and in the end, this may work out to be a benefit. I expected that with a 33" waist, the blue portion would wrap farther and the strap wouldn't touch my mid-section. Moving the clip all the way to the side is much more comfortable than clipping in the middle.
I have found that I get a much better workout when tethered. Instead of ordering the tether, I just use a nylon rope and a carabineer. This allows me to work in a smaller space and focus purely on the workout instead of trying to zig-zag between the swimmers and the water aerobics class. It is also easier to concentrate on running form, which in my case, needs a lot of work.
The workouts have been great. It is very easy to get a good workout and adjust the intensity to any level desired. I am using a plan designed for runners recovering from stress fractures. As they point out, it is important to do more interval training in the water to get the same benefits as running on land. I would also suggest getting a good water-resistant watch with a large digital display.
I definitely recommend this product, but I would suggest checking with your local rec center or water aerobics instructor to see if you can try one out first to make sure it works for your body type/shape. They make several models for a reason. Getting the right fit and buoyancy is key.