I must have had mima for about 3-4 years now.
With a good standard setup I saw about 5% improvement, but I was real hypermiler before so my gains were smaller than the average driver.
With a (phev) higher capacity battery it's a doddle to achieve huge numbers (150mpg) until your battery is exhausted. However there is a delicate balance to be struck to ensure maximum life from the batteries and maximum mpg over a journey. Also the IC engine has to be allowed to warm up to the lean burn point or it has a serious impact on mpg. This warming period for every journey consumes a certain amount of fuel and in the winter mpg goes down as the warmup period is longer and the heater may be in use. A preheater helps with this as would a thermos system if I ever get it built!
MIMA is a great mod. Highly recommended and prepares you for a bigger pack one day perhaps
