The demo day was back in Jan or Feb but it was a one-time thing. Go for the rental instead. Again, don't borrow one unless the person is willing to adjust it to fit you. The initial fitting can take some time. My wife and I took over an hour to dial it in just right. The greatest thing about bp/w is that they are "one" with your body with no flopping, but ONLY if they are properly fitted. A shop like Brownies has great service and will fit you perfectly. That is the only way to really know the full difference between this and a regular BC.
The bp/w will be different from what you are used to. There are no pockets, no "cummerbund" no big padded straps. It is completely clean, and with most of the weight (the plate) and all the buoyancy on your back having even level position in the water is almost effortless. And, yes, you can float upright on the surface fine and swimming on your back is even easier.
Like I said, we never went back. My wife is newly certified, and I am very advanced, but even she saw the benefit right away, and after having achieved perfect bouyancy and trim in 12 feet of water (the hardest depth), she got out and said, "I want one of these!" It is not just for "tech" divers at all.
However, I don't want to seem like a "this is the only way" type of person. I do suggest you spend 25 or even 50 (for two days) and try it out. At least then you will know what you like.