If you want reassurance about how unlimited Plusnet Unlimited is check out this post: http://forums.thinkbroadband.com/plusnet/t/4341189-r...
There's no issue with traffic prioritisation (it only comes in to play when you are maxing out your line - your speeds are not affected by other users (excluding BT bottlenecks which affect all ISPs except LLU)), limits on how much you download (I'm expecting more posts from people shifting > 1TB per month) or prety much any other technical aspects.
Where there is an issue is the length of the phone call queues and the time they are taking to respond to tickets. Plusnet customer numbers are growing faster than their support can cope with at present. They have a new support centre in Leeds but it's taking time for the situation to improve.
Summary: when it works it's great, if you have issues it can be frustrating at present.
Plusnet user since November 2001 - not sure for how much longer