I tried bikes which had kits added to them.
http://www.bionx.ca/en/main/default/31.shtml
I tried the PL250 and the PL350. And the plastic Chinese pedal/scooter thing, but, no.
I probably wouldn't get one with regen because what smart driver/rider uses regen?
Also it adds another layer of complexity. One more thing to go wrong (and in fact the 350 needed to be "rebooted" because I noticed while riding it that the brake lever activated regen wasn't coming on).
If I were going to buy one, I'd probably lean towards a brushless DC system with "pedal start" - (vs. throttle start from a stop). It's the simplest version of the most efficient motor style (vs. brushed DC).
Then you have to decide what battery chemistry you want, based on price, range & weight.