X-Plane is not open-source, though it is available on Windows, OSX, and Linux (perhaps the last bit is causing confusion). Flightgear is the big open source flight simulator. I haven't really messed with it recently - it is REALLY lacking in UI. The simulator engine itself seems decent, but it is a real pain to set up each flight and unless something has changed just moving to another airport involves exiting/restarting. That said, the last time I looked at it somebody was working on a pretty serious free 737 implementation (FMS, many systems, etc) - not really comparable to the high-end proprietary FSX aircraft but it was making fast progress and it was FOSS.
I made the switch to X-Plane somewhat grudgingly. It certainly has elements that are superior to FSX, but it is far behind in ATC and traffic generation (not that FSX is spectacular, but at least it has some kind of ATC, routing, auto-tuning, etc), and the UI is somewhat less friendly (especially around flight setup). The GPS isn't quite as capable either, not that FSX is really spectacular there either. The main thing X-Plane has going for it is that it hasn't been abandoned - sooner or later everybody is probably going to need to make the switch so I figured the time was right. X-Plane also handles multiple CPUs and additional RAM far better - FSX really gets unstable when you load it up with textures, complex aircraft, etc. I don't think I've ever seen X-Plane crash, and now it is 64-bit so complex textures/etc should be even less of an issue.
However, much of my early concern with X-Plane was more in the context of using it for recreation. As a flight training aid I think it is at least as useful as FSX. I'm not really using the radios at all in the sim as neither sim is terribly realistic in that area - I'd probably use something like Pilotedge if I wanted radio practice. Likewise I'm not so likely to be jumping from place to place and so on - I'm more likely to just set up at an airport and fly around locally as I would while learning. But I don't really want to get into the whole debate about whether flight sims are useful for instruction in the first place - my personal feeling is that they have areas where they are useful but you need to understand their limitations and not rely on them to get the feel of flying.