Hello everyone,,
I've not used it in a while, but in my experience it's really good and really... I hate to say bad, but bad.
Good: As a programmer I love its open source and development-oriented nature. It provides a really easy to use scripting language for interfacing software/hardware with the sim. I have an RC controller and an interface to plug it into my computer via USB, but with the way it reports controls (everything is an axis, even the three switches) I needed some scripting to get it to work as I wanted. Because of the easy-to-use scripting language with good documentation (although IIRC I had to guess at a few things) this did not take long. I also like the fact that some of the instruments (the wet compass in particular on the default Skyhawk) look better than their counterparts in FSX and the controls are a bit more intuitive (for those without radio stack controllers, press a button and you get a menu into which you can type in frequencies). The 3D viewing options are pretty cool as well, but I've only used the red/blue mode and with moderate at best success.
Bad: I haven't used it in some time so these might have changed, but I found the flight dynamics to feel very simple and unrealistic. I believe I still have it installed on my laptop (albeit an old version) so I could get back to you on that with more, but I remember it not being all that fun to use because it didn't feel anything like flying a real airplane. The (graphics) models also seem incredibly simple, and I don't seem to recall a time when I was able to get it to load textures properly; something was always blank or fuzzy or had some texture it shouldn't. In short, it feels sloppy.
I would say it makes a fine procedure trainer - if you're looking to practice using VORs or practice flows in a Cessna 172, you don't need anything fancier. Beyond that, I wouldn't use it for all that much.