Just finished my Cyclone build and took it for it's first ride yesterday! Thanks everyone who contributed to the discussion here. Some really great ideas.
I'm pretty cheap, so my goal was minimum cost for a decent range mid-drive. I don't do any crazy MTB trail riding, mainly just rail-to-trail type paths with my wife or old logging roads. The donor is a cheap walmart 27.5"+ bike that cost $120. I know it's terrible and will probably break soon. I'll be working on my next bike in the meantime. The cyclone kit from Luna Cycles was $350. I built the battery pack out of 18650 cells from new laptop batteries. The cells are all Samsung 2.8Ah cells configured in 10p20s. So the pack comes out to 72V, 28Ah. Right around 2kWh. Total cell cost was $240, plus a 80A Chinese BMS.
Total cost: around $800.
Holy crap, this thing is fun! Why did no one tell me this existed sooner?! It's like a dirtbike that I can ride wherever I want! I had previously ridden a 1000W hub motor, and there is absolutely no comparison between the two. This would probably beat my KLR 650 to 35 mph, and I really don't want to go any faster than that on this bike. It goes up hills like they aren't even there. I actually have to concentrate to keep the front wheel on the ground, even at 10-15 mph. I haven't ridden long enough to make any range claims. But even using 40Wh/mile, I should be able to hit 50 miles easily, which is further than I ever ride.
My biggest gripe is the throttle. There's about 0.001 degrees of rotation that corresponds to 0-100% power. Everything else in the throttle rotation range is just dead. So it's either full throttle or no throttle, which is probably why I can't keep my front wheel on the ground, particularly from a standstill. I'm going to add a potentiometer onto the GND side to try to eliminate the large dead zone.
Next is some kind of dashboard. I don't really like the look of the Cycle Analyst, and it's pretty pricey for what it is (I'm cheap, remember). I'm thinking about making an Android dashboard app that communicates with an Arduino either via BT or USB OTG for all the voltage, current, speed, temperature information... I know there's been several projects like this, but they're all either dead in development or not very DIY friendly. My plan would be to get a $50 Kindle Fire Tablet for a screen.