Over recent years I've always used the blue Bosch range with Ni-cad batteries... they have been totaly reliable and very sturdy... I could'nt really fault them.
But after using a friends 18v Hitachi combi with Li-on batteries, I'm afraid I've now changed brands.
The Hitachi is so light compared to the Bosch, and the Li-on batteries seem to go on forever. I recently installed 60 fire signs in one day. Most of them required the hammer drill....all were done with one battery.
Unlike the Ni-cads the Li-on batteries dont slow down as they discharge ... you seem to get full power and then all of a sudden they just stop.
Definitely wont go back to Ni-cads.