From what I have heard, nobody has much experience yet with the Kelly controllers. They do look to be reasonably priced AND offer regen.
Series Wound DC motors aren't ideal for use as a generator. Some of the other styles of motors work well though.
Mr. Sharkey's Electric Rabbit uses a shunt-wound motor and features regenerative breaking.
I have heard that permanent magnet DC motors also work well for generation.
Regenerative braking is NOT high on my list of priorities. As this will be my first electric car conversion, I am looking for cheap/easy/uncomplicated.
Now if I can end up easily adding regen brakes, I definately would.