The MixCubes are very good.
@Razor - it was that review that made me try the C50 rather than the C5. I have no issue with the fact that spending more money might give better results, but was giving advice based on the fact that you were working on a budget.
For the money, I DO think the C50 gives you a good alternative/ different "view" of a mix. I have certainly found my mixes improve when A/B-ing with a pair of decent nearfields against a single C50.