Ball valves are great! However, they are designed to be either on or off. If you try to use them to control the pressure and flow, the inside brass will erode. I've seen leaking brass erode to the point of no return in a week. Once brass erodes, it gets worse fast.
I've been using plastic ball valves on my hose ends to control on/off, and to a limited extent, flow. Plastic does not erode like brass. I used the Gilmour inline and the doubler. Here are pictures of both...
http://www.gilmour.com/Watering_Hose_En ... t-offs.asp
I love these things. They are super easy to turn on and off, and when I replaced the stinkin flat washer with an o-ring washer, they don't leak. In fact, you might put one of these on your hose bib and control the hose flow with that.