Just got my IMC rating (well, had to tell someone!)
and found FS2000 invaluable.
I could set up my local field (EGNF) with clag at 1000' and practice tracking VORs, NDBs, ILS approaches to my heart's content.
Of course if you've done it first for real you know where your weaknesses lie and can then practice just those bits without the cost of real flying. But it really helped with instrument approaches as I could practice all the local approaches before the test so I knew the headings and profile - this really reduces workload in the real thing. After all, in your course you are unlikely to do more than a couple of ILS approaces unless your home field has ILS (or other instrument approaches for that matter) which most don't.
If I'm flying anywhere else for an instrument approach in future I'll try it out on FS first to get some familiarity before doing the real thing.