Depends what software you use. Mostly, when you want to stop a RAM disk, it dumps everything onto an ISO file so that tou can mount it again next time you boot up your computer. That's why I said, the boot time and shut down times are increased.
For anyone who is interested, I did a little benchmark with a 4GB RAM drive: My WD caviar black scored 120 MB/s in sequential read/write speeds, whereas the RAMdisk scored 6500 MB/s in read and 9000 MB/s in writeing speed.