I usually use a merc on all my chars, if only for a couple hundred to a thousand extra dmg. My WW barbarian, however, uses my favorite mercenary of all. With a 4.1k stone, andys, and an Insight partizan, he could tank better than my barb. Crude, I know, but effective.
Like Bloz, I can't afford the expensive stuff on NL, so I generally mf a Bonehew and pop a shael or two in it, or if I'm really lucky a Viperfork with a pRuby for some differing elemental damage.
Didn't think anyone else played NL, Bloz.