The cost for the amount of energy stored is very high. Even as a suitable buffer would be very expensive for an EV. If you need to store electricity quickly for regen, and are using NiMH or lithium-ion or any chemistry that can't accept charge quickly without hurting life, than it would probably be best just to get a small lead-acid pack. AGMs, especially hawkers can be charged extremely quickly for regen for short periods, and can deliver energy quickly as well.
One of the reasons the cost is so high for ultracapacitors as a suitable buffer is the discharge profile. Energy stored = 1/2 * C * V^2 This tells you the energy remaining is related to the voltage squared. So to get 1/2 the energy out, you have to go from 100% OCV to about 71% OCV. If you want to get 3/4 of the enrgy out you have to go to 50% OCV. This is different from a battery which delivers most of its energy at a more stable voltage. Nonetheless if you can find some caps cheaply they might work well. I don't know if the price is comming down a lot now, or if you can get some surplus from the government.
