It depends on who you ask. I really love Warren Brown's IMBC recipe which you can find on YouTube. I prefer this because it's easy to make, much less messy than a PS-based icing, and it has a very elegant mouth feel. It doesn't taste cheap or sugary. It's very silky and smooth.
Flavoring is easy. It's just like regular BC. Add extracts, oils, nut pastes, liqueurs, fruit purees, etc. It's also easy to decorate with, although it is more susceptible to melting from the heat of your hand. For that reason, I usually have another back-up bag filled and just switch the tips until the other bag "cools off" a bit (you can also just put it in a cake pan filled with ice).
With meringue buttercreams, the texture is manipulated with temperature, not the addition of liquids or more PS. So yes, you can pop your bag in the fridge to stiffen the BC and do some roses easily enough. No need to add anything to thicken/thin it. Also, it works fine under fondant. You just have to chill the cake well and cover it while it's still cold.
1. Look up the Warren Brown recipe on YouTube (I add about 1/4 to 1/2 cup extra sugar)
2. Always use unsalted butter. I use Land O Lakes just because I don't have the money for Plugra and it's the best I can find where I live
3. Use GOOD flavorings, and use a lot. I use two to three TBS of Nielsen Massey vanilla extract. Please, for the love of all that is holy, don't ever use clear vanilla or imitation vanilla in this BC.
This is a website I always like to show people when they are reluctant to use meringue buttercreams for decorating. These folks use SMBC I think, but IMBC is basically the same thing, just different in preparation.