Depends on what you are doing. If you are aiming for the clubs I would say something with a good low end is essential, the Mackie HR's are well regarded in that sense to the point I've heard them described as having something of an over hyped bass sound. Otherwise, I guess what you like the sound of. Beyond a certain price point you are getting technically good monitors, the differences are down to personal taste.
I personally mix on Tannoy Reveals with a separate amp. Next set will probably be either Genelec 1040, KRK V8, Mackie HR824 or Adam A7. And I mixed some good stuff on old hifi speakers before I got the monitors. Key is always knowing how things sound on them and how things translate to other systems. Reference mixes here are a great help...ie: a CD of other mixes you want to sound like. Also critical listening on a variety of systems to check mixes out.