I had 6600 (with 5600 shifters). I like full 5800 a bit better; hood shape, braking, cables, and shifting (especially in the front). Also have 5700 now on the latest 'rain/beater' I picked up, and I'd probably slot that just behind the 6600. I'll likely run the polymer cables on it eventually.
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"The older you do get, the more rules they're gonna try to get you to follow. You just gotta keep livin', man, L-I-V-I-N." - Wooderson
'14 carbon Synapse - '12 CAAD 10 5 - '99 Gary Fisher Big Sur