I've heard the M-Audio ones that are pictured up there (can't remember the model name now, AV40s?). I do some work with a guy that uses them to mix and uses them to practice his DJing in the spare time when hes not recording.
They pack a wallop definitely, loud as hell - Stereo field is fairly broad so projection isn't an issue which makes them fairly easy to get positioned when your using them as near field monitors. As far as frequency response they're pretty flat across the board with a slight bump in the low mid end which isn't noticeable but does warm them up a bit, I tend to like that and as a pair of speakers I think they sound really good for the money. Cons are that the amp they use gets very hot which always worries me about the longevity of them and general build quality also, the blue light really pisses me off when you have them on stands at head height, so I tend to put a post it note on the volume knorb and status lights to filter out the light - my eyes get really fatigued quite quickly if I don't.
In that price range I'd suggest the Fostex PM-04's though:
They got really high ratings last year from loads of magazines and a good few industry rewards as memory serves. Build like a tank, same as KRK for build quality and have a separate amp for each speaker which is nice. There flatter in the frequency response but have a much nicer tweeter, makes guitar mixing just that little bit nicer IMO. It has bigger brothers if you looking for a bit more oomph and bass response but I really like these speakers. Make sure you get the MkII version - it's a lot better than the first version.
Other than that go for NS-10r's or clone of. They are the standard, everyone uses them and for good reason.