NTL was rock-solid for over a year. Then Virgin bought them out in Feb 2007. I play in a serious end-game raiding guild and have had to cancel my place on all future raids due to constant random disconnects and massive lag caused by whatever Virgin has done to the service. From the day Virgin took over my latency has increased from a normal 30ms to between 500ms and even 5000ms.
I strongly recommend that you look elsewhere if your connection to the Internet has to be reliable and stable.
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