Anderton
Guitarpima
Anyone else got tips for use of MBCs?
Got two for you.
1. Don't think of it as a dynamics processor. Think of it as a five-band graphic EQ which lets you vary dynamics within a particular frequency range. For example, you can isolate just the bass frequencies and compress them, while applying no dynamics to the midrange and just raising or lowering the level of that band somewhat...while expanding the high frequencies just a teeny bit so they aren't overbearing. Remember, most multi-band "compressors" are really multi-band dynamics processors, so you can expand as well as compress.
2. As crossovers for multi-band processing. I described this in my Advanced Workshop video in the section on multi-band distortion, but the important point is not to use any compression, e.g., ratio of 1:1.
I always start with no dynamics and apply dynamics only where necessary. I rarely use dynamics in all the bands. FYI many "maximizers" are really multi-band compressors, they just hide that behind the interface to make it easy for people to squash their mixes 
The Concrete Limiter is an exception, it's stereo.
+1000 and how I use it as well. One of the things guitarists with high gain may experience is "whoomfing". 8 out of 10 times you can curb this with high passing. However, certain notes while chugging will lash out with a "whoomf" type sound. A Multi-band will control this so good, you can actually leave your bass a bit hotter if need be and the whoomfing will be gone. This "whoomf" thing usually happens on A, Bb and B chords that you chug on for metal guitar.
Another use....say you have a bass guitar with a low B. And say any time that low B hits in the song, it lashes out louder and makes its presence known a bit more. You can automate there....or you can select the frequency via multi-band, set your threshold to police that frequency on the bass and call it a day.
A MB will also do wonders on mastering when you just need to tighten up some low end after you've finalized your eq or if you find a mix has some sub low frequencies that just need to be controlled in spots. The best thing about the MB is you shouldn't have to use much to notice a difference. Lower ratios work really well (2:1 and 4:1) and like Craig mentioned, you don't need to mess with all bands. This is where people using Ozone mess things up. They grab a preset and tweak it until it sounds good not realizing that there are times when you don't have to touch everything to make a difference for the better.
To be honest, I don't use them to compress at all. For me, they are a great way to control mixes or instruments that sound good for the majority that may get a bit strange in spots. Set the MB to police any spots that push, and it will control things beautifully without you even knowing it's working. :)
-Danny