I have both in my systems (128MB Mushkin rev2 and 256MB Crucial cl2)... from what I noticed, like everyone already stated, the mushkin outclocks the crucial... but hell for the price of rev2 v. CL2 its hard to justify if your on a tight budget, i was at the time of my 256mb purchase... Anyway, both ram vendors are great and their products are probably the best in the industry, its just they cater to different crowds and needs. Mushkin = Overclocking, Curcial = Decent overclocking (good in some cases) and great prices...
anyway, thats just how I see it...