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My Laserjet P2015 does 1200x1200 dpi. The text looks very crisp and clean.
I'm testing a Brother HL 2170w (non-PostScript), which costs far less than my Laserjet, yet it boasts an "HQ2400" dpi of 2400x600.
Text printed in this 2400x600 mode looks quite a bit thicker and darker, but I'm not sure if that's a good thing. Frankly, I think this 2400x600 feature is rather dubious, and I often see it on low-cost/budget lasers.
The odd thing is that the Brother's 600 dpi output looks nearly the same as my Laserjet's 1200 dpi output, although the Laserjet's text looks slightly cleaner. Mind you, OS X HP drivers have never been stellar.
What's this gimmicky 2400x600 dpi all about? I also noticed that a couple of sites seem to understand it as 600x2400.