I installed the EV kit from coastaletech yesterday. When I used it for the first time, I had 7 or 8 bars of energy. I only got a little over a mile when it got down to two bars (admittedly, I had to go up a hill at 30mph, so I'm sure that was a drain on the battery). If I were on flat land, I'm sure it would have lasted a little longer.
One thing I did notice was that once it got down to 2 bars, and the ICE kicked in to re-charge the battery, it wouldn't stop. Even after the bars had gotten back up to 7 or 8, whenever I tried to coast or feather the gas pedal (and would usually be able to engage only the electric motor), the gas engine would always engage. Almost like it was "stuck" in a recharging mode. When I drove later in the day (after stopping and re-starting), I didn't encounter that.
I think it's like $40. I nice little investment over the long term for driving in a shopping center, or a few "stop/go" drives between malls or stores.
I installed it myself (I'm no tech!). It was pretty easy, although my biggest piece of advice is to make sure you have the one wire pushed ALL THE WAY into the Prius' wiring harness. There are three wires - two hook to "taps" you put on Prius wires, the other is inserted into a harness. I thought it was in all the way, but it had to go in about a millimeter more before it would work. This was on the THIRD time I had to take everything apart because it wasn't "working."