MTB outfitters in Bali - Bali Trailblazers
I've ridden with them and I recommend their services. A good resource if you are in Bali on holiday and want to get out and do some "real" mountainbiking.
I rode with a guide from Bali Trailblazers and agree with Jeff, they are a good outfit. The guide is a freelancer and also leads rides with Bali Cycling, website below; I haven't tried Bali Cycling but the guide Katut was a good friendly guy who was happy to tailor a ride to keep me happy :-)