Weaknesses: Cranks too long. Cheap shifter and derailleurs. Website spec misleading (front crank has 38t and 28t, not 42t and 34t). Still too heavy.
This review is on 10' model Ninja (which has exact same spec as girl model Jem). I have been planing on purchasing the best quality 20" bike like Specialized or Kona for my son as his 6th birthday approaching....My search is over when this lightly used Ninja bike showed up on Craigslist. The quick release hubs first caught my eyes as none of the other companies offer quick release hubs in this 20" wheel size category. I took off the front shifter & derailleur to reduce bike weight down to 23 lb. After a quick tune up and test ride, my 8 y/o daughter found this Ninja runs quicker and smoother than her own Hotrock.
Of course, my son is very happy as he can now shift simplified 6 gears (instead of 12) to go on many local trails with me and his big sister. I am impressed with the bike's built quality and cool paint job...but also wish the crank can be shorter like 120mm to accommodate average 6 and 7 y/o kids legs.
This bike won my 5 chilis for best 20" kids mountain bike available, in my opinion. After remove front shifter & derailleur, it is much lighter than cheap box store bikes, heavy double suspensions or quirky disc-brake equipped junk. Kids need higher quality bikes, not cheap heavies with tons of useless gears. Improve the rear shifter and derailleur will make this an unbeatable 'Ninja'. Highly recommended for every bike loving boy.