I'm very fond of FMS, and I use it a lot especially in the winter. Unfortunately it isn't a truly plug and play piece of software and requires some effort on the part of the user.
The first problem is finding a particular model you might want to fly. If you can't find one that is already written, you have to use the specialized software, there are links on the FMS site, to make your own. This isn't a simple process, and it requires a fair amount of information to create a properly flying model. This is where the rub is. You can find several versions of the same model, but they likely won't fly the same due to being written by different authors. If you want a model to perform correctly, you'll probably have to build it yourself.
The second problem is that FMS isn't supported by a staff of programmers. Any updates to FMS, like new versions, aren't likely to come along too frequently. However, I can say the latest version is quite adequate.
Next, if you have any problems, of any kind, the best you'll be able to do is ask your question at one of the forums at the site. The likelihood of getting a good answer is slim to none!
Finally, there are many ways to interface the program. This will become quite evident is you notice the names of the various forums. I ended up designing my own interface, and I have a couple of posts that describe it:
Oh the Joy! More Joy!
If you're interested in a very good and simple USB 3-axis interface, I can sell you a new CH products controller.
Incidentally, I found exactly the model that I fly, the H9 Ultra Stick
40, already written. It performs identically to the actual model, so I didn't have to create one. I would recommend it. One last item is that any model needs to be trimmed out just like you would have to do if you had a new model at the flying field.
In summary, FMS is an excellent flight simulator that require a bit of thought on the part of the user. Anyone who complains about it probably wants a video game.