I found parallels to be a great application. I started working on a project were I needed two servers each running MySQL (Master/Slave replication) and a Web Server on the Slave. Unfortunately, the hardware was not going to be in until 1-2 months into the project.
I now use parallels to run 2 CentOS Servers with MySQL Master-Slave Replication between them and a Web Server on the Slave. At the same time I also run Windows XP to test my Web Application with IE6, IE7, Firefox, Opera etc.. under Windows.
Not to mention I have Photoshop CS3 + Textmate + Navicat + iTunes + etc... running at the same time.
My machine is iMac 2.0GHz Core Duo with 2GB of Memory - and it's fantastic!
I have had no slow downs - and everything just works!
Parallels allowed me to "virtualize" all my work without the need to have the hardware with me!