I can talk a little about this, I too have a bunch of apps in the appstore using MonoTouch (like Quicklytics, githubby, and a bunch of other ones for clients).

I can talk a little about this, I too have a bunch of apps in the appstore using MonoTouch (like Quicklytics, githubby, and a bunch of other ones for clients). Performance of monotouch is extremely good. For 99% of what you're doing, you do not notice any performance loss at all. I've played around with disabling GC temporarily, but I haven't seen yet a reason to do that in my apps. A few of them use things like huge databases with data, creating thousands and thousands of gc'd objects, and everything works amazingly well. Not everything can be done in MonoTouch, though. For some things you'll never get enough performance unless you go to C. You can't do a lot of things in Obj-C either. Remember, Obj-c is a dynamic language, so every method call is fairly expensive, which is not the case when you're in .NET land.