The battery charge doesn't just come from regenerative braking - it actually charges the battery anyway using the engine if it's running. For example, when starting from cold with the battery depleted (from the end of the previous journey), the battery is fully charged within about five minutes or so. This doesn't seem particularly efficient, although it's how trains work so maybe it's better than it sounds
Electric power does make quite a difference to journey MPG. On my commute to work, which is four miles urban and then eight miles fast rural roads, it does 28-30 MPG (adjusted from indicated figure). Because the engine is cold, it's using petrol for nearly all of the urban part, and after that I'm doing 60-70 anyway. On the way home though, battery power is sufficient for nearly all of the four miles urban at the end, giving 35-37 MPG for the journey. I guess what the hybrid part does is introduce a much greater variability to the MPG for any given journey. If I lived half a mile closer to work my MPG for the homeward journey would exceed 40.
Economy aside (as I was expecting about 30 anyway despite the grand claims), my other beefs are with the 8 speed auto. In comfort mode, cruising behind someone at 50, flooring the accelerator to overtake does not do what I would expect. There's a delay of maybe a second before anything at all happens. Then it drops down some gears, then some more. It's maybe 1.5-2 seconds before you get full acceleration, which doesn't seem right to me? It's not really conducive to safe overtaking. Then there's Sport mode, which doesn't do it for me either. Why, with an 8 speed box, would you ever want it to be doing 2500 RPM at 30 on almost no throttle at all in town? Why not do what AMG/Jaguar etc do, which is still be doing 1250 RPM, but downshift as soon as you use more throttle, and not until then. It's a bit perplexing