A week or so ago skadamo brought the Zero DS to my attention.  After checking out Zero's website (including a video of Jay Leno checking out the S model) I checked their "Demo Rides" section to see how far I'd have to go to check one of these bikes out.  Shockingly, there was a dealer just down the road, about 40 minutes away!  I immediately emailed them to get their hours, and was able to sneak out there for a few minutes last Thursday night.
At the moment they had only the Model X in stock, so my test ride would be limited to the parking lot (when they get a Model S in stock I'll be going back for a longer test-ride and posting a more thorough review).  For now, here's my impressions of the Model X.
The Model Xl has a few settings that I think are unique to its off-road-only application.  First there is a "training wheels" switch that limits the amount of power available at the throttle and a speed limiter which, when engaged, keeps the top speed within "electric scooter" limits.  The sales rep. (Bryan Muehleis) turned both of these on for the first part of my ride and pointed out several dings and a bent handlebar as evidence of previous test riders who underestimated the bike with the governors disabled.

Interestingly enough, with the governors enabled, the bike performed similar to the "normal" mode of other electric vehicles I've tested.  The nice thing about having this option is that you get a chance to get used to the other aspects of the bikes handling (very useful for me because I've spent almost no time on MX bikes) without having to concentrate on throttle control.
The bike is direct-drive, which means no shifting and no clutch.  Anyone who's driven a manual-transmission vehicle is familiar with modulating power using clutch slip (even if you're no longer consciously aware of it) and the lack of this control is one of the first things I noticed pulling the bike away from a stop.  Even with reduced power, the bike jumps away from a stop with no hesitation (there is a touch of play in the throttle, I'm not sure if that's intentional or a side-effect of this being a "demo bike").

A few other oddities of the Model X can be found in the brake setup.  As you'd expect the bike has hydraulic discs up front and in back as well, but the size of the controls is more bicycle like than motorcycle.  Part of this feeling for me comes from my regular bike, a Honda Magna with oversized controls but even compared to Jamie's 250 Comet these controls feel small (maybe this is a typical motocross thing?).  In any event they appeared to work just fine (as much as I could push them in a parking lot) and I imagine their mass (or lack of) contributes positively to the bikes exceptionally low weight (about 150 lbs).  The other item of note regarding the brakes is that both brakes are activated from the handlebar, and there is no foot pedal for the rear brake.  Not that I make much use of it, but it was a bit unnerving coming to my first stop and feeling nothing but air under my right foot.
After a few laps with the limiters on I stopped the bike (you have to turn the ignition off for the changes to take effect, a nice way to prevent accidental (de)activation) and disabled the governors.  Pulling away with the power limit removed, I could see how you could get yourself in trouble if you weren't prepared.

With full power enabled, the bike is quick; I'd say it pulls away from a stop at least as hard as my M750 Monster and with the light weight combined with max torque at zero rpm, keeping the front end on the ground requires discipline and conscious effort.  After getting used to the power, I gave it a healthy dose of throttle on the back part of the lot and felt the front end get loose; more than enough notice that I need not attempt to feed it full throttle in this enclosed space.
After returning the bike to Bryan, he eagerly mounted it and proceeded to thrash around the lot and the lawn of the place, clearly having established an ad-hoc off-road track before.  It was particularly cool to see someone clearly familiar with off-road riding give the bike a thrashing and I can only imagine how much fun it would be on a real mx course or even some trails through the woods.  The best part is that, with it's light weight, low seat height (for a dirt bike) and configurable power delivery it would probably make an ideal bike for a beginner, which I happen to be.
I'm looking forward to doing a more thorough test of what Zero has to offer when Pro Motorsports has an "S" model available to test on the street.  In particular, it will be interesting to see what the bike feels like at the advertised top speed of 60mph (which I've heard can be changed, in either direction, via gear changes or possible software control).  The street bike is heavier (larger battery pack) but it's still light for a motorcycle and it will be interesting to see how it handles at speed.
In the meantime I need to go rename my "guitar jar" to a "Zero jar", and start checking soda machines for forgotten change…