I've been interested in Boosters as well since I just got my first tube amp, since apparently boosters only work well with tube amps. I like the sound Tony Iommi gets on a lot of his solos, and I heard its from a Booster. Fireball Ministry seem to use them a lot too. As far as how much you'd use one, depends on what you are trying to do and your style of playing....The Booster I am interested in a lot is made by Vintage Tone Project, but I know Build Your Own Clone offers a kit for about $90 with all the parts, just solder it yourself. Also, Keeleys Java Boost has the 44s in them...
I ended up buying a pre-built BYOC Tri-Boost off of some guy on another forum for $70 shipped. Which is nice considering the unbuilt kit is usually like $90.
Anyway I will post a review on this sometime soon for anyone who is interested.