I put them in most of mine. They work great and give good peace of mind and early warning if things are bad.

On my giant scale stuff with the lipos at 6V I use the I4C digital meters.

http://www.i4cproducts.com/r_c_applicaltions.htm

http://www.i4cproducts.com/c-volt.htm

A bit more expensive but on a $2000+ plane it's well worth it.