I'm on the BE/O2 16 package with Xilo and have been since December, think they now refer to it as Elite.
I have a long line and the difference in stability between O2 and BT 21CN line management is like night and day.
Zen did a lot to try and help me with the connection but because of the line length the DLM kept hammering the connection speed.
I too had to think a little about the migration cost but decided to got for it and am glad I did.
I know have a rock solid connection running at just over 3.6 Meg which will happily support VPN which I have to sometimes use.
I've dealt with both Matt and Tom at Xilo who have been extremely helpful.
I am definitely an extremely satisfied customer
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen => Freeola => Vivaciti (using O2 Wholesale DSL) => Xilo (C&W Wholesale) => Xilo (O2 Wholesale)
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.