My single largest dollar investment in tools are 6mm hex driver bits of various kinds, shapes, and sizes. I have at least three lengths of bare hex bit, bit sockets in short, med, and long, with most duplicated in straight and ball end, as well as plain old drivers with handles like a screwdriver. I have Wiha bits, Snapon, Stahlwille, Hazet, Craftsmen, and Harbor Freight. Don't spend money if you don't have to, for most users the HF works fine.
The HF set of 100 different bits is my favorite. When a bit costs 8 cents I don't even hesitate to hammer it into a munged bolt head, and 99% of the trouble when using a hex bit is that it wasn't in deep enough.
Lots of times I don't like the wiggle you get from using extensions, so an extra long bit or bit socket is nice to have.