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.
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