Ah, the standard Zen myth typically propagated by Zen users. Actually you'll get comparable service from other ISPs - for less (ex-Zen user).
As someone who has recently migrated from a premium expensive ISP (Eclipse Internet) to a really cheap one (TalkTalk), i can relate to this. My service is identical bar the ability to get straight through to tech support (how often do you need that?).
My opinion is that bandwidth is simply a commodity these days, get it as cheaply as you can. A lot of the issues above will be backhaul congestion which will affect all BTW isp's. If the issue is ISP specific simply move to another big cheap ISP.