PimpRig has posted a review on the LS Cable LSHS L500 and X500 CPU Coolers

I could come up with only 3 reasons why nickel is used on the bottom of heatpipe heatsinks: Nickel bonds very well and having a nickel bottom would protect the surface, prevent oxidation, and diffuse the heat. Since nickel retains heat longer, the heat will spread over a larger area which spreads it evenly across the heatpipes so you don't have just one or two doing all the work. I don't know if this is actually the case, but it makes sense to me (unfortunately, so does beer flavored ice cream).

LS Cable LSHS L500 and X500 CPU Coolers Review