Well according to ppl on adsl guide Plusnet is not that great. They have a lot of traffic shaping (slowing down internet traffic thats not html mostly), reports of bad customer service etc.. Although i doubt it will be that bad unless you are a heavy user or expect very high speed.
As to the guy saying F2S getting taken over by plusnet, someone already said it was actually pipex who took them over, and Pipex are in a completely different ball-park, they are much more established, and a higher quality service.
Just stick with plusnet for a bit, and if they are starting up noticable problems for you then move (unless you are still in contract, in which cas you only have a few weeks from their policyu changes to move!!)