As a huge fan of the RF 55g, and a huge non-fan of the 45g RF, I will say that the 45g switches on the 660C are actually really, really nice to type on. There's a crispness and tactility to them that I didn't find on either the HHKB or the 45g RF. I have no clue why it is, but there ya go.
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I've heard this from another guy on GH. After owning two 45-gram 87Us and replacing them with 55g RF KBs, I'm intrigued about how the FC660C manages to produce a different feel from Topre's 45g switch. There's no doubt on my part, if this keyboard was 55 grams, it'd be on my desk alongside my 87UB. Thanks for the information, daerid.