upgrading to hps, some ?'s

Hey, I have a couple grows under my belt using cfl's and was recently thinking of upgrading my lighting to a hps setup.

I was looking into a 400w hps (concluded that it would be the most efficient for the space im working with). I was wondering if anyone thought this was a good deal and if not point me in the right direction! thanks!

http://www.amazon.com/Watt-Electronic-Ballast-Hydroponic-Light/dp/B002J7PYSU