I prefer running Win7 in VMWare Fusion rather than Boot Camp. I can hot-key between my Mac and PC environments, copy/paste between them, etc. It's just so nice to have my Mac be a Mac... and also a PC.
If you do heavy gaming in Windows, you'll want to give it all the resources of your machine (meaning: Boot Camp), but in my development work I do not notice a performance hit.
Battery life of a laptop will be somewhat reduced by running a virtual machine, since the processor is working harder. But again, the impact is small.
A major benefit of going virtual machine vs. Boot Camp: Time Machine will back up your virtual machines. It won't touch a Boot Camp partition.
What I do: I keep a bunch of virtual machines on an external hard disk, one of those cool little USB affairs. That way I can keep my projects separate, avoid version-itis, etc. Works really well. (Actually, my main VM pocket-drive is a speedier little FireWire-equipped unit, but the firewire connector is so easily dislodged from the side of my Mac that I really can't recommend it unless you're really really careful. Honestly, whoever designed that connector really needs to be taken to the woodshed. It needs to be several mm deeper to provide a solid connection. You do NOT want your drive disconnecting in the middle of using a virtual machine!)
If you go the external-drive route: first format the drive using Mac OS X Extended (journaled) format. (Don't use the silly crapware utilities that might come bundled with the drive. They're universally terrible.) This allows Time Machine to back up the drive, which is a good thing. External drives are excluded from Time Machine backups by default, so go into Time Machine's preferences and un-exclude it.
I've had better luck with VMWare than Parallels. For one thing, though neither company is a champ in customer support, few are as abysmal as Parallels. There is a free alternative, VirtualBox from Sun/Oracle, but it's not really ready for primetime in my experience, lacking graphical and USB support.
VMWare lets you clone an existing PC, which is very nice, too. I think Parallels has this capability as well.
Enjoy! Virtual machines are great for exploring other OSes. I've got a collection of Linux and other VMs too... even tried Android!