Well, I also used PlusNet for a while, and I found them OK. The problem my cousin is having is high contention I think. She is in a highly populated area of South London, and finds her connection very slow at peak times, but OK at, say, 5 in the morning! She also has big problems with random disconnections, and has been unsuccessful in trying to get support from PlusNet, as the support lines are seemingly undermanned. She waited on hold on one occasion for 35 minutes, before giving up, so you can see why she might be reluctant to stick with PlusNet.
Basically, she wants a faster connection, but only uses the Internet occasionally, and is understandably a bit reluctant to quadruple her monthly Internet expenditure, on top of the initial set up costs. She's never actually experienced Broadband, but I'm sure, when she does, she won't regret it!