BEST: As much as I like all the 105 goodies I upgraded to, I would have to say best purchase was Look pedals and the Shimano shoes to use them with.
WORST: Piece of c__p Specialized Afterburner double blinkie light. TWENTY BUCKS and the tiny, flimsy molded in (ie, not repairable) clip broke off the very first time I tried to clip it on a strap. I didn't even really need the thing and bought it just because I was at the LBS and had been thinking about getting an extra. It uses the same little batteries that computers use, which I didn't care for; I prefer standard AA and AAA in such things. Now I buy $9-10 5-LED blinkies from Performance and Nashbar. Bought two from each place. They are the exact same blinkie. Nice robust clips and they work great. Use 2 AAs.
The broken blinkie paid dividends, though. I had it rubber-banded to my rack. At an outdoor concert a couple of weeks ago, a just-learning-to-walk toddler was fascinated with my bike. I reached over and turned that blinkie on, and he seemed to experience a state of ecstacy. I took it off and let him have it to play with, still blinking. He went around showing it to everyone. It would have been worth $20 just to see that face.
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If it ain't broke, mess with it anyway!