The more I use Parallels the more I prefer it over rebooting into Boot Camp. From a speed perspective I know that Boot Camp has to be “faster” however for most things I do on Windows, virtualization is usually sufficient.
I am sure that part of this is that my new 27″ iMac with 8GB of RAM is now fast enough to support what I do in OS X and run Parallels at the same time. With my older iMac I would normally run Parallels, but would avoid doing too much else on OS X as it would be rather slow. The new iMac is so much more powerful and faster.
I am in the main using Windows 7, though now and again I do boot into Windows XP.
If you are using Boot Camp, I would recommend you give Parallels a go.