I haven't used in in quite a while simply because I don't have any time frames where I am guaranteed to be uninterrupted lately, but I have never had a bad experience on the network.

Granted, you have to have a very good idea of what you are doing before you go online, but I would be willing to say that almost all of the controllers now are very professional, pleasant entities to interact with. Also, in the unlikely event that you come across a controller that gives you a hard time, you have the option to report them to both the VATSIM Admins, and their regional ATC branch supervisor.

Other than the sometimes low traffic levels, and the requirement for text speech when a party is not able to use voice, plus the ATC coverage, the rest of the experience is pretty much the same thing you would get in real life. Real world procedures are followed based on the newest charts and diagrams.

The only thing I can think of that might dissuade some people from trying is the following:

Most controllers will try their absolute best to accommodate you if you have outdated scenery, charts, etc, but they have the final say on any action that should be taken in their airspace.

Other than informing them (preferably in the flightplan notes when you file) that you are using default scenery or old charts, you generally have to do what they say, even if it is not as commonly done in the real world. They will try to follow real world procedures, but I would advise against arguing against minor semantics if something does not 100% match up to the real world.

The relationship between ATC and pilots on this network requires mutual understanding, cool tempers, and frankly, teamwork, for a flight to be successful and enjoyable. I've heard stories from both sides that this or that ATC/Pilot came in thinking they were the absolute expert on everything, and refused to bend in their judgement and practices. It just doesn't work like that.

All in all, I hope you give it a shot, but before you do, read the rules, tips, and help sections so you don't go online absolutely clueless. Also, understand that everyone online is learning new things everyday, and that you must be willing to adapt to different ways of doing things sometimes, although for the most part, it will be 100% accurate.
It's a very fun system to use!

Enjoy! 

most importantly... do they give unrestricted climbs...