I know you're looking for views from sellers, but I had to just jump in and say: as a buyer, I am seriously unimpressed.
We looked at two houses with PB; one we tried to submit an offer for but we were dealing directly with the vendor who had no clue the stamp duty rules had changed and was trying to finagle some weird deal with us that wasn't going to fly ("you pay me 249,999 for the house so that saves you 5k in stamp duty and then you can buy the fixtures and fittings - which by the way were fitted wardrobes - for 5k cash"...tried to explain to her she was only saving us 250 quid under the new rules, not 5k, but she wouldn't have it and was insistent she wanted 5k cash for fixtures and fittings either way. We walked off. House is still up for sale, now at a reduced price which is the same amount we offered her....!
As an aside, the online 'make an offer' thing is horrible, it's akin to submitting a sealed bid. If I'm putting an offer on a house I want to be able to talk to someone and explain my own position, and judge from their response whether it's worthwhile me putting in another offer etc. I dislike being forced to just type a number in a box and then being told 'yes' or 'no' with no context or feedback, and without the vendor being able to see whether I'm a better buyer than the next chap. We actually went round to the house and knocked on the door to talk to the vendor when we wanted to put an offer in...
Second house is just around the corner from us, we know the houses and area very well, and it's overpriced by about 25 grand. We went to view, the chap who showed us around knew absolutely nothing about the property (told us "I just open up and lock up") and wasn't willing to find anything out for us, told us to contact their Head Office. I did, with several questions, they were supposed to call me back but never did; I then tried emailing the 'local property expert' (whom i later found out covers the entire region by himself - what the?!) who didn't respond for over a week; he only responded then because I happened to be talking to my hairdresser about it and it turns out she knows the property expert's Mum and called her for me.....! He only answered half my questions so i went back and re-asked the remaining questions, that was over 2 weeks ago and nothing. DH and I just can't be bothered to chase this any more and are taking the 'it just isn't meant to be' route, though I'm tempted to stick a note through the vendors' door to let them know their agent is pants.
There are two other houses in our (small) village up with PB; both went up at 300k which is just too much; two weeks later both dropped by 20k, and then another two weeks later both dropped by another 20k. Like clockwork - on exactly the same day. Hmmm.
As an observer, seems to me they're trying to flood the market and take over just by getting as many vendors as possible on their books. They probably do this by over-valuing. They then do b*gger all to help you sell your house. If it hasn't magically sold itself in a short space of time they'll tell you to drop the price. Then wait and see if it sells itself again. And when it doesn't, tell you to drop your price. And so on....
As buyers, we now avoid any houses listed with PB. I can't imagine that they are particularly good at keeping the chain and buying/selling process alive and on track, based on my experience of even trying to get an offer sorted out!