I very rarely get client-side lag. I do, in certain places, get video lag. My video card, despite being I thought fairly decent when I got it, is on Blizzard's low-end of usable cards. (Or maybe that was Sony's for EQ2, I forget now.)
There is, right now, a fair amount of server-side lag. This is an issue that I'm sure is absolute priority one for Blizzard, as since the last patch there's been a few crashes. However, prior to the last patch, for the two or three months I was playing before that, the servers have been overall remarkably stable.
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Only the exceptions can be exceptional.