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Just got myself a new Panny P42ST60 for £570 - albeit only with 2 year warranty. Pretty chuffed with that. Will be my first plasma! I played around with one at my local panasonic store and just had to have it! Even though I didn't buy it from them (much too expensive there: £850....) They let me load up quake live and if I'm honest I really couldn't feel much lag wise. It felt fine playing. I'm guessing if you're competitive in any sort of way it would be a bother but if not then I don't see the problem.
Care to divulge where you bought it?