I recently bought one of the cheap ones off ebay (£122) from Pirana Trading, this included an additional extension to take it to a 16ft reach. It seemed really hard going to begin with but once we'd dismantled and greased it up it was great.
Like most most good new plant the more you use it the more you realise just how good it is. Our first job using it was on a 13ft high vaulted ceiling and it was invaluable.
I'm not saying for one minute that they are as quick on a normal job as just just pinning the board up with your head, but for a very occasional tacker or on awkward jobs, you can tack all day and still feel fine at the end of the days work which to me is £122 well spent.