I use them, I think they're great. Probably one of the the Top 10 inventions that have led to better biking.
I have two. A slimer model for shorter trips and a bigger bag with lots more junk for the longer trips.
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger