Having browsed considerably about fans, my first question would be whether you're getting any problems with engine temp control. If not, I'd take some convincing that there's anything that needs replacing.
My own issue was simply the sound of the fan; see thread on The Howling. The viscous clutch on my 560SL allows the fan to be turned fairly easily by hand with the engine off, but there's no trouble keeping temp within spec in stop-and-go traffic at 85F ambient.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)