I thought I'd follow up with a report, since I did rent a bp/w on my holiday diving trip. It was a Halcyon Eclipse with a Regular-sized stainless-steel backplate and a 30# wing. Just the basic webbing harness, and no extra pad or anything on the bp. I had a 3mm jumpsuit wetsuit/1mm vest on underneath it.
First of all, it fit me amazingly better than a jacket BC, right off the bat. It actually felt like part of me, instead of like a clump of unruly pillows.
The first couple of days were spent in a shallow pool of about 5' depth (I was doing a refresher OW class). I didn't notice any strap discomfort in the pool, but at home that evening I could find two sore spots on my back/top-of-hip area, where I guess the bottom center part of the plate contacted it. I think that's because most of the time in the pool I was standing up on the bottom, or kneeling, so it was being worn like a backpack, with a lot of gravity, instead of how it would more typically be underwater. In any case it was not sore enough to really bother me, just to notice it if I pressed there on purpose.
On my first day's ocean dives with it, I wore a separate weight belt with 13# of lead weights. The DM wanted me to have the weights in the back so they did not interfere with the releases in front. Things were generally good - but I did notice a couple of issues:
1) I needed to get most of the air out of the BC in order to descend and to stay down, and it was a bit difficult. I had to kind of "milk" the BC. I'm sure most of that was due to my beginner techniques and positioning in the water, but it did seem more difficult than on previous jacket-type BCs. I was told that the Halcyon rental wings did not have a bottom dump (although later I think I might have seen one but it was just a thread with no grasper dongle - I'm not sure about that though, as I might be thinking of one of the ones they had available for purchase). At any rate, I effectively did not have any air dump except the inflator/deflator hose on top of the wing.
2) I was constantly fighting being rolled over onto my back. That is, if I paused and did not do something to counteract it, I would turn over like a sad turtle, instantly. This led to a lot of "fighting" it with finning, arm waving, etc. Again, I'm sure a lot of that was due to my newbiness. I'm sure I looked like an idiot
On the next day, I decided to try putting some of the extra (weight belt) weights in the front. I made sure the buckle was still accessible. I also added two pounds. (I realize that probably overweighted me, but I was trying to work with the equipment I had; there were not weights in the crate to equal out to exactly 13#.) I did think that maybe if I didn't have to have *every last* bit of air out of the wing in order to be neutral, then it might be easier. I will have to research more, but I think that perhaps I would be happy with a smaller wing and with a bottom dump. Of course also my technique can improve! (It did just in the course of the two weeks.)
So, I had about 5# just on each side of the front weight belt buckle, and about 2# around in back on each side. This made it *much* easier to not turn turtle, and I felt more neutral front-to-back. Since the waist strap of the BC was at my real waist, I wore the weight belt lower, but still above my hips so no chance of slipping off that way. I also figured out that I could put the weight belt on first, then put on the BC, and then just slip the "coming up from below" part of the BC's crotch strap under the weight belt as I donned it. That was much easier than trying to get the belt on after the BC!
I think I got better at angling myself properly to let air out of the wing, and also with the extra weight I did not need it to be *totally* empty in order to be neutral. (That said, I was not so weighted that I could not have swum up with a failed BC; although, as I said, if I had my own gear, and better skills, I could eliminate a good bit of weight and feel more comfortable with the wing's performance.)
I did find a new problem though, which was that the top of the wing was riding so high that the tank was again slopping around like it was not even a part of me (the jacket BCs did this constantly). I went back and consulted with the shop, and they showed me how I could tighten the shoulders to help eliminate that (I had chosen to loosen them slightly in the beginning, as the D-ring gliders were kind of digging into my arm/chest).
I say "help" eliminate that, as there was still an issue that the bp was just too long. That is, the waist strap goes through the plate at a fixed point, and so the amount of plate that extends above it is also fixed and can't be "shortened." I did try on a size "Small" Halcyon plate at the shop, and that fit me really well and eliminated this problem completely; but, understandably, they rent the "Regular" sized plate. I think I would have purchased the Small plate on the spot except that it was aluminum, and I (think I) want stainless steel.
Another thing I noticed was that it seemed to be more difficult to control my buoyancy. I had got to the point previously where, while I was no expert, I could keep myself at one level (more or less) as I was swimming along. I seemed to have regressed with the bp/w. I think it felt like it was harder to "fine tune" the air in the wing with the inflator. Like it was more difficult to let a tiny puff in or out. I did get better at it as the dives progressed, and of course if I could just use lungs that eliminated the problem. I wonder if a smaller wing would help with that as well? At any rate it was not "bad" enough to make me want to go back to the jacket.
I also at first did not feel as comfortable on the surface with the bp/w set up. I'm a good swimmer, and comfortable in the water, but I have tended to feel a bit panicky when I'm on the surface prior to a dive. With the bp/w it seemed a bit magnified. But, then I discovered that I could just flip back and float on my back, and what a difference that made! To the point where, on our last dive when we had conditions that were super sloppy on the surface, with big waves, current, etc. I was able to spend 15 minutes floating on the tag line waiting for my buddy, and I felt totally calm (his regulator flooded with water when he got in, so they had to do something with it, then he got in again and it happened again and at that point he got a completely different reg put on).
Let's see, what else...
Oh yes, I did the doff/don exercise in the pool, and had no problems with the continuous, one-piece strap. Yes, I had to contort my shoulder a little bit, but it seemed much easier than the other folks who were struggling with the various buckles and straps that were loose and fluttering as they tried to re-don. It worked fine for me, anyway. I don't have the greatest shoulder mobility either, due to various injuries and strains over the years. I liked the simplicity. Just see the open loops and put yourself in them. Then the main part is on and you can take a bit more time getting the crotch strap in place.
Speaking of which, the crotch strap ruled! No more endless creeping up of the BC!
I never did rent a jacket BC on this trip, although I did go in to Diver's Direct and try on a few of the women's jacket BCs, just to see how they felt this time, compared to a couple of years ago before I had tried the bp/w.
Well, even in a size Small (and I'm not usually a Small!), they just did not fit right. It's the short-waisted thing. I think it's partly because the cummerbund is so wide (i.e. measured top to bottom). My waist is above my hips - that is not negotiable. Therefore, the bottom of the cummerbund has to sit at the top of my hips. With that in place and tightened up, there was a gap between the top of my shoulders and the shoulders of the BC of inches! Good old BC around my ears....
So, in summary, I really liked the bp/w. It fit me *much* better than any jacket BC (and I tried on many women's jacket BCs), and I liked the simplicity and the adjustability. Also I really loved the lack of bulk and fouf. Much closer to the fish feeling
I have the feeling that my own bp/w, with a small plate and (maybe?) a smaller wing, I would like it even more. Also then I could have my own places to secure the computer, the inflator hose, and etc.
Thanks for the advice here, and especially for cluing me in to the fact that I could rent a bp/w

Now if/when I do buy one, I'll have a better idea of what I want or don't want.
B.