The Honda 600 had a 600cc engine, hence the name. There was also a smaller engined variant, the Honda 360, but that was hardly sold in the US. I wouldn't say the Fit looks retro at all; both the hood and windshield are very sloped and overall the proportions are quite modern. The 600 is more like a Mini (and in fact similar in size), with a pronounced break between the very horizontal hood and nearly vertical windshield.
I like the Fit a lot, much more than the CRV. The CRV is very tall, which is really detrimental to both aerodymanics and the center of gravity. Over 50 mph aerodymanic drag starts to become more important than weight in affecting fuel mileage, so it will never be as economical as it could be at highway speeds. A high center of gravity does bad things to both ride and handling. For off-road use the height and ground clearance are advantageous, but from what I've seen the CRV isn't built for heavy duty use anyway so I miss the point of all the height.