Everything is a giant con! To be honest, I'm starting to think I could do without the drama of changing providers. Pipex might be sh*ts as a company, but at the moment, I can't fault the service, and better the devil you know...
That said, I am quite attracted by Zen, and the 20Gb limit wouldn't be a problem for me. I would rather have a constant fast connection, without peaks and troughs, but I don't think I would get a faster connection than I have with Pipex, because BT haven't upgraded the lines in my area yet, so we are only averaging between 2 and 4Mbps at best.