You don't have to have the line rental with PlusNet even for you to have Fibre. Remember it's Openreach engineers who would be installing whether you are with BT or PN. I don't know exactly what packages you are comparing (up to 40 GB usage or 'unlimited' vs 250 GB) but in general the PN prices are lower and even if you have to pay the installation fee, the difference in price would mean any short term savings with BT would likely cancel out after a while.
I know there's a 50 quid Sainsburys voucher at present but can't help thinking that BT is being a bit sneaky as several of their charges go up early in January 2013. I know PN's fee (for line rental) went up recently, but you can stick with your existing service if you wish. I use Primus for the phone line as it is somewhat cheaper than BT (and I make calls using 1899.com at 5p per call to UK landlines, no matter how long, and from 1p/min for calls abroad).
As an existing customer the installation for Fibre is 25 quid (I think) as I have been looking at getting a second line and fibre (possibly both from PlusNet, though they were offering 3 months for a fiver a month for new customers recently, but no similar deal for loyal existing customer [have been with them for 8 years or so, apart from a while when there was no landline in one property I lived in a while back]). As Primus charges less for line rental and only 59 (vs 49 by PN) for a new line, I may get my second line from Primus and then add Fibre when service is available (postponed in this area from September to December).
As for customer service, any growing ISP sometimes has complaints, and PN is no different (though still regularly winning awards). If you look at the PlusNet section on the Thinkbroadband.com forum, you'll see some complaints, but there's also a small group of PN staff in tech support who try to sort out long-standing complaints. It's often the case someone posts there with their complaints when something has gone massively wrong, but the staff do (from time to time) acknowledge when something could have been handled better, and sometimes can offer goodwill in the form of discounts, or refunds when service was at fault.
When I moved here it was for higher speed but I must be in one of the few streets on Merseyside where Virgin don't supply cable/fibre... Really annoying as they send all the mailshots despite the immediate vicinity not getting service. Much as I'd like to have tried Virgin, seems like Fibre when it eventually arrives will be the service I can use to give me speeds of 30+ Mbps. 80 feet away the homes can use Virgin...
Line of sight at front and back, but nothing doing in this nice little cul-de-sac :(