I've got a couple of the green Photon II's. The green should help preserve night vision, and the battery will last for over 7 days straight! The Photon II's have an instant on pressure switch as well as a continuous on switch. I carry one on my keychain, and my wife carries one also. Everyone that sees the Photons just loves them. I picked up some for stocking stuffers. They're well worth the price IMHO.
Here is a great link for Photon info. It explains what the different colors are good for, battery life for each color, etc... A great resource:
Here's a few links from TFL's sister forum, BladeForums.com, discussing Photon usage:
Hope this helps.