This hybrid system could work, but not just as a bolt-on for people on this site.
Most hybrids' main benefit is from downsizing the engine close to the minimum required for cruising (or running Atkinson cycle) while maintaining reasonable power on reserve for passing. Simply bolting on an electric motor will not do this. However, electric assist could, for the average driver, prevent some degree of downshifting, or allow for lower gearing with the same drivability. Although I imagine most on this site would give up a bit of rate of acceleration for more economy, everyone has their limits, and that extra bit of torque could make "excessively" tall gearing more bearable.
On my Insight, electric assist makes available around 30% more torque in the rev-range I typically run. Because of this, I can keep up with traffic with the little 1L engine without ever revving over 2000rpm. It produces as much or more torque than Honda's 1.6-1.8L engines at these low RPMs.
The next biggest benefit is auto-stop, which many of us here do already with kill switches or key-off, but a hybrid system makes it a lot less trouble to use. In my Insight, I shift into neutral and tap a button, and the engine dies. When I'm ready for the engine to come back on, I tap that button again or throw it into gear and it springs to life silently, without putting any wear on a starter motor or clutch. I can't stress how luxurious it is for the engine to auto-stop at a redlight or when rolling down a hill and start itself back up without the need of reaching for the key or bump-starting it. More than any other feature, I'd miss this one the most if I ever went back to a non-hybrid gasoline vehicle.
Regenerative braking probably doesn't account for all that much in terms of economy for most of us here, but would for a more typical driver. For me, the biggest benefit is that I can go longer on a set of brake pads - I swapped out my factory pads just past 200k and they still had plenty of life in them.
Some smaller benefits: Most hybrid systems can more efficiently produce 12v than an alternator, even with a DC-DC converter, and for non-belt hybrids, get rid of some parasitic losses from belts, so you have close to the same benefits as an alternator delete without actually giving up your ability to charge the 12v battery.