I have more hours spent flying on VATSIM than I am prepared to admit on an internet forum...
I use X-Plane for fun. And for me, it's not fun unless there are other people on the radio. That's where VATSIM comes in. Is it realistic - pretty much. They controllers do a good job replicating real-world procedures and use the right phraseology. Several of the center and approach controllers are real-world controllers and the service from them is excellent. The virtual-pilots - meh. It's hit or miss. Anyone can pop on VATSIM and try to fly in the virtual system. Some are good at it. Some not so much. But the ones who can't follow instructions or are in way over their heads with aircraft they are trying to virtually fly make it pretty amusing to listen to.
What I usually do is log in during the evening hours and use the map.vatsim.net site to pick an area with good coverage. Then plan a flight around that. Even better is to find an event night where entire sectors are staffed up from top down. I did one this past week-end from LA - Denver with fully staffed atc from Clearance Delivery in LA (yes, you have to file a flight plan and get a clearance. And they check them to make sure they are valid) all the way to ramp control to the gate in Denver... ok now I am outing too much to this group, but hey it's a cheap hobby and you can enjoy a few adult beverages while flying in the virtual system in VR on a A321 ok...
Would I use VATSIM for training... I don't know. I never did. Like I said, I just do it for fun. Honestly, it's a bit addicting. Once you fly with virtual ATC, it's hard to use a simulator without it.