I use the Rose Levy Berenbaum "Mousseline Buttercream" recipe, which is an IMBC. Her recipe calls for cream of tartar. I have no idea why though!
. I am told that IMBC is slightly more stable in the heat (meaning, it lasts an extra degree or two) than SMBC. I agree with Jamie- it's worth the babying. The other downside is that you can't get super-vivid, deep colors so you must cover with fondant if you are doing, say, a red cake. To which I say: who cares?! It's worth it
Jamie I like your tip about using cold butter. I admit, every time it's turned to soup and I've had to chill it to revive it, it's been with room temp butter. Obviously the whole thing wants to be colder.
rbuckhoff, if you want to firm it up a bit for piping, just fill your icing bag and then pop it in the fridge for a short bit.