Playing Modern Warfare 2 regularly I now don't notice any lagging at all. When compared to a 0.4Mbps Sky service, it really is incomparable. Previously, if anyone else in the house was using the Internet; messaging or whatever, you really couldn't play, but now you can throw anything at it and the gaming isn't affected at all.
I've found I'm more likely to host games based on MW2 matchmaking, but mostly only if there are British and European gamers in the party - very few games have ended because an existing host quit out - I would regularly take over as the host. Ping tests I've done have shown up excellent results - but there's still the necessary latency to get across the water to US gamers.
This shows the major benefit of FTTC to me - much more headroom to do more individual things at the same time. Only once did I actually see close to the maximum speeds on a single download - 4.5Mb/s - and iPlayer, XBox and PS3 downloads go very fast. But you're more likely see quick speeds spread across a number of services, rather than one service taking up the whole.