There's a little bit of lag, but no less than using the LG Plex client that's built into the TV.

I installed raspbmc onto a USB stick, so the OS is running from that it's definately quicker and less laggy than running it from SD. I know that things can be further speeded up by enabling turbo mode, but I didn't have the time to fiddle and it just made the thing reboot randomly.

I wasn't convinced about using the Pi as a media centre in the early days....I'm much more convinced now, it's pretty good (if your TV supports CEC or you have some other IR setup).

I can't say specifically about whether CEC lags, I don't have another remote control or USB keyboard in which to use the GUI on it, but it's perfectly usable - not apple tv fluid, but it's prettier.