When I recently purchased a Macbook Pro, I had a couple of options to set up my .Net development environment on there. I could use a Virtual Machine based solution such as Parallels or VMware Fusion, or run Windows natively using BootCamp. My first preference was to stay inside the Mac OS if possible. I spent some time researching which product other developers are having more success with and Fusion seemed to be more stable and responsive for the majority of them. I downloaded the trial version from their website and took it for a spin, and have been really happy with it.
I have used both XP and Windows 7 in a VM with Fusion, and the performance has been pretty flawless. I allocate both processors, 40 GB of hard disk space and 2 GB of RAM to a VM. Some of my peers have long been advocating the advantages of developing using VMs, and now I totally get how cool this way of working is. I can create and restore snapshots at any point of time, create a new VM to try out any beta software, and keep my development environment isolated from softwares/utilities that it does not need (avoiding software bloat). I am using a Windows 7 VM right now for the most part and playing around with Visual Studio 2010 on it. I intially found the Unity feature in Fusion (it gives the illusion of running Windows apps natively on the Mac OS) to be really cool, but I don't really use it much. I have dedicated a Space to my VMs, and tend to work in full screen mode.
If you end up going the same way as me, I would recommend looking at multiple vendors before purchasing Fusion. I bought if off Amazon, and it cost me $23.49 (including a $10 mail-in rebate). Other sites,including VMware, offer it for upto $80.