I can't comment on the likes of Vivaciti as I've never been a customer but I did have a brush with O2's broadband division having been a happy O2 mobile customer for a good while, unfortunately, the difference between mobile customer service and ADSL customer service was like night and day with the latter being the night, not pleasant at all.
Being a tad geeky I will tend to look for a network topology diagram on an ISP's site which indicates they own, control and develop their own network (rather than reselling wholesale services which seems to be common these days) and also in Zen's case it shows the capacity of the links and external connections which is as equally importatnt as the last mile to your home, I was impresed with what I saw, along with the fact they didn't traffic manage (I'd been with another ISP that is advertising quite heavily with a value proposition at the moment and found myself with an unusable connection, it cost me dear to buy myself out of the contract)
With Zen the connection just works in the main and when I've had any cause to call them its always been a pleasant experience even though I've had to wait quite a while to speak to someone on occasions.
Last year I wanted to see if I could get anymore speed from my line which still had an extension style socket on it from when it was converted from an extension as the BT bloke that came in 2004 to connect the new line couldn't be bothered to fit an NTE.
I'd moved the line to Zen's tlephone package to make any problem resolution easier but was fully prepared to have to pay for the socket change as it would obviously require a visit from Openreach (OR),
I had originally broached the situation on this forum and had received some help.
To my surpirse the tech support guy said that he would be specifially requesting a broadband engineer from OR so that I wouldn't be charged.
The guy came, was very pleasant, changed the socket and thoroughly tested the line, later in the day the fault tracker showed the person I'd been communicating with via TBB had checked the line and reset the profile to take advantage of the increased sync, I suspect with a good proportion of other ISPs I might have had to ring and request that.
Furthermore, when the VAT increase was pending I mailed and asked whether there was anything they could do.
We managed to come to a mutually agreeable price, again I doubt I could do that with a lot of ISPs.
I'd also argue that while you maybe able to prepurchase additional bandwifh from some ISPs, those that allow it to roll over until used are in the minority, this is what I have done.
Perdonally I'd not recommend any other ISP and don't intend on moving. while definitely not the cheapest I would say they are the best.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen Lite 8000
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.