I am looking to purchase a BMS 150 for my grandchild.Does anyone own one of these? sure would like to know of any problems I can run into.  

With chinese atv's you can run into alot of problems.
they are cheaper and the honest truth is that they are cheaply made as well.

and it doesn't take long for them to wear out.
i haven't seen one at my that lasted a single season without needing major repairs.

I think it's safe to say that you would come out alot better buying a good used yamaha, Honda, suzuki, or kawasaki.
they know how to make these machines so they'll hold together.