A 2013 Zero DS was on the USA's Big Dog Adventure Ride 2013 last summer. It did as well as the full-on 640cc or larger dirt bikes over some high altitude rugged stuff. Pilot technical skill was a contributor to it's successfully competing in the event. Owner used it half a day, charged the battery while having lunch, used it the second half day. Two oddities were not having a shift lever to down shift (no gearbox) and no engine noise.
Where I really can see these carving a new class is in the super light category.. Mixing up the downhill MTB type vibe with a power system and cutting in on the KTM freeride type of the market.