MIMA can use the full assist to climb many hills at 100MPG, which sounds great, but the dark side is that while you are charging back up, your MPG will suffer because charging presents a load on the engine, unless you charge slowly on level ground, or more aggressively going down hill or coming to a stop. The trick with MIMA, is to only use as much assist as necessary to improve on the hill climbing MPG, and then to carefully put the charge back when it will have the least impact on MPG. If your driving is on the flatlands, MIMA may offer only limited improvement in MPG, but if you drive on rolling hills, in the mountains, and other less flat conditions, MIMA with a driving technique that uses the new level of control carefully will improve your mileage.
The best way to get a feeling for the benefits of MIMA will be to read about the other peoples experiences with MIMA on the MIMA users thread at Insight Central or on this site on the users comments page.
We are currently looking at adding a grid chargable booster battery, that will charge the stock pack while running, without the need to rob power from the gas engine. This will allow more aggressive assist, with the resultant improvement in MPG, but will not depend as heavily on retrieving the charge from the gas engine. The results should be impressive.