I'm using it for well over 2 years without any problems.
Just make sure you make regular backups of your virtual drive in case something happens.
I'm running Win XP, Vista and Linux on the same machine. No problems so far.
I'd also recommend to try Fusion as well. Some say it's a more stable/reliable application, but Parallels has far more features which I prefer.
If you have any specific concerns -- please tell.