O2

The price and support from O2 is excellent.My speed is not O2's fault(Long line),plus no cable,fibre available.Getting just over 2.5mb.Been with O2 now nearly 3 years,still no sign of my exchange being upgraded to FTTC so no chance of O2 offering me anything faster.
When my contract is up in July I'm off onto the Digital Region project where I can get roughly 37mb with a FTTC connection

I've been with O2 for a year and a half now, after jumping ship from the godawful over-subscribed behemoth that was Virgin Media.
I have the home broadband 'All Rounder' package, which with a £5 discount for having an O2 mobile, costs me £23 a month (was £20, but the price went up recently.) For this I consistently get around 10MB broadband (2-3 times better than the best I got with Virgin's fluctuating speeds) which is good considering I'm 2 miles from the exchange and plenty for my needs,
and no frills home phone, without having to fork out to BT for their over-priced line rental.

O2 keep you informed about service changes, billing and rare outtages (proactively - I received an apology for loss of service before I'd noticed there was one!)

Customer service is good. My broadband stopped working recently and I couldn't find the cause, so I contacted O2 customer services on their freephone number and after the standard automated selection menus I got straight through to a friendly Northern Irish support lady. I was expecting to have to wade through the usual bunch of stupid questions "have I tried turning it off and on again", but she got straight to the nub of the problem with minimal preamble. After determining my account was fine and the adsl at the exchange wasn't the issue, she traced the problem to the router and said they'd send me a new one (would I like the new upgraded model, or did I have a sentimental attachment to the older existing one? I opted for new and improved.) The call to customer services was at 5:25pm, by 10am the next day the new router had arrived by courier. Plugged it in and was back online in under 5 minutes.

Their account management website is easy to navigate and gives you plenty of info and options (compare with the labrynthine complexity and negligible accessibility of the BT account site.) Their 'My O2' connection monitoring/support software has a minimal footprint and can be uninstalled without impact on the service, but I find it comes in useful to resolve occasional connection hang-ups.

Overall I'm happy with O2, certainly a damn sight more than I ever was with Tiscali/Virgin or BT.

I was an O2 mobile customer when O2 supplied my broadband, so I was always able to call their customer service team for free.

This was useful because the phone line in the house at the time was not the best owing to some "creative" alterations to the wiring inflicted upon it by the previous inhabitants.

O2's support team were always very efficient and NOT an Indian call centre to boot. And for this, O2 deserves credit.

The connection speed was always pretty good. I don't think they provided many if any other services, not that it bothered me.

Would recommend.

When I first joined o2 I was getting between 14 and 15meg download, I live within two miles of the exchange and just recently my speed has dropped to between 5 and 8 meg download speed.

Went online chat with an o2 advisor who said my router was continually disconnecting, he also said he could find no other problems so a new router would be sent out to me and will arrive in 48hrs, now that service.

Will update this review if the router solves my speed problems.

EDIT: New router arrived, still low speeds.

Summary: On old terms, happy with service, worried throttling for new customers is a sign they don't expect the network to cope with many more customers.

Been with o2 since I escaped the hell of Tiscali in 2007.
o2 have provided a consistently good, fast, hassle free, genuinely unlimited service ever since. Very pleased.

Gripes
Very short lived connection problems very occasionally.
The supplied Thompson router's configuration options have been artificially limited and If I set a port to open it seems to reset itself to closed shortly afterwards but this is not enough of an issue for me to have tried to find a solution.
Usually get 6 to 8Mb peak times on a 16Mb contract but fast enough for everything I do.

Overall
Good, reliable service. £18 a month just for broadband is a little steep but is just about worth it in return for a hassle free service. I may leave as I feel I'm wasting money still paying line rental to BT but I will not sign up to o2's bundled packages as the p2p throttling is extreme and it may be a sign they are worried the network will reach breaking point if they get many more users. I hope that isn't the case and they offer an unlimited bundled package with the quality of connection I've grown used to before I jump ship because I am a touch nervous of getting into bed with anyone new in case they as as bad as Tiscali were.
I have marked services and support as average as I've never used them.