I'm commuting every day from Southend on Sea to London, and I've had BT Mobile ever since it launched. For the first year, I was absolutely happy (apart from the reception hole in Chalkwell).
The last few months, speed has deteriorated massively, though. Despite a solid 4G signal, it often takes ages to open a single mail, connect to a text-only chat service, open Facebook, load new tweets on twitter, etc
Data transmission rates actually seem okay once a connection is established, but a ping of 600-1100 is definitely not normal, and there's a lot of packet losses. That looks to me like the network is more limited by simultaneous connections rather than bandwidth, which is weird, as the network could handle chock-full commuter trains without any issues, and now I'm even having trouble accessing my mails on an off-peak train.
Has anything changed a few months ago? I would really like to see it go back to normal?..