Inspired by Sound on Sound's Mike Senior, I got one of these monitors. It is the single most useful mixing tool I have ever bought.
I use only one, mixing in mono, thus avoiding the combfiltering inherent in a stereo setup.
The Mixcube gives a very good picture of what is going on in your mix. Combine a Cube with headphone monitoring and you really can make some good mixes. Before I got the Cube, I wasn't able to do that.
Granted, bass levels have to be checked elsewhere. But that's about it.