I've been using MonoTouch for a few months now, I ported my half finished app from ObjectiveC so I could support Android at some point in the future.
Here's my experience:
Bad bits:
Xamarin Studio. Indie developers such as myself are forced into using Xamarin Studio. It is getting better every week, the developers are very active on the forums identifying and fixing bugs, but it's still very slow, frequently hangs, has a lot of bugs and debugging is pretty slow also.
Build times. Building my large (linked) app to debug on a device can take a few minutes, this is compared to XCode which deploys almost immediately. Building for the simulator (non-linked) is a bit quicker.
MonoTouch issues. I've experienced memory leak issues caused by the event handling, and have had to put in some pretty ugly workarounds to prevent the leaks, such as attaching and detaching events when entering and leaving views. The Xamarin developers are actively looking into issues like this.
3rd party libraries. I've spent quite a time converting/binding ObjectiveC libraries to use in my app, although this is getting better with automated software such as Objective Sharpie.
Larger binaries. This doesn't really bother me but thought I'd mention it. IMO a couple of extra Mb is nothing these days.
Good bits:
Multi-platform. My friend is happily creating an Android version of my app from my core codebase, we're developing in parallel and are committing to a remote Git repository on Dropbox, it's going well.
.Net. Working in C# .Net is much nicer than Objective C IMO.
MonoTouch. Pretty much everything in iOS is mirrored in .Net and it's fairly straight forward to get things working.
Xamarin. You can see that these guys are really working to improve everything, making development smoother and easier.
I definitely recommend Xamarin for cross platform development, especially if you have the money to use the Business or Enterprise editions that work with Visual Studio.
If you're solely creating an iPhone app that will never be needed on another platform, and you're an Indie developer, I'd stick with XCode and Objective C for now.
