Packet loss always the problem for me
I'm on VM and have experienced huge problems with packet loss and jitter which I've really struggled to explain to their support staff. On the surface the download speeds and latency seem fine, which is all they seem to care about, however any kind of 'real-time' application such as VOIP, gaming etc really suffers.
Having spoken to about 10 different support workers in India who all seem convinced that Internet Explorer settings are to blame (despite the same problems persisting across Linux and iOS devices) I've pretty much given up. The problem does seem to have lessened recently, so maybe VM have started addressing some of their capacity issues.