Some... I've had no problems running some of my older Python programs under it. About the only thing I had to change was some database connection stuff and it all just worked. I've run it under .NET and mono, but never in a production capacity.
They are two different implementations of the same language. One of the key differences is that IronPython can compile Python to IL and be run natively on .NET whereas CPython is compiled to its own bytecode and executed by the CPython runtime. CPython has a C API and has tons of modules available that are not available on IronPython. IronPython does not have the GIL, so multithreaded apps have a chance at running faster there. And, under IronPython, Python programs will have access to all of the .NET libraries. In general, CPython is much faster than IronPython.
CPython is much more portable that IronPython and has fewer dependencies. The number of IronPython users is quite small compared to the number of CPython users. If you run into a problem with the language or library, you will have to duplicate the problem on CPython because that is the reference implementation.