I don't. Not yet anyway.
a) I'm not smart enough to use one properly yet and b) I'm kind of under the impression that type of thing is unnecessary if whatever is feeding the master bus is already sounding good so I'm trying to focus more on what comes BEFORE the master. Then on the master I just do some VERY light compression (to kind of glue everything together) and extremely mild EQing (like a slight hi shelf boost or cut as needed and mid range cuts/boosts to get rid of/accent stuff). For that I've just been using the PC2A (just because it's easy to use and sounds cool) and Quadcurve EQ (easy to use, right on the track strip). As I said it's SOOOO light all it's really meant for is a touch of sweetening and cohesion so I don't even want to touch those big bad buggers (and every time I do I don't like the results anyway). Then at the end of the chain is the Concrete Limiter to get things a bit louder and prevent overages.
But I'm not mastering and really at best I could only produce a half decent "demo" quality mix. From what I understand that type of multiband stuff at the mastering stage is to fix problems so I'm trying to learn to avoid those problems in the first place.
I actually think once I start getting into multiband compression stuff I'm going to use the Sonitus Multiband. It's lightweight and I'm really not certain those big fancy MBC's are going to produce much better results anyway. At least not at my skill level (which is middling at best).