Oh, I forgot to mention...I have a rack on my other bike because I regularly carry a pannier on it and it only gets used for rain rides or commuting. This bike is my 'go fast' road bike, but it's also super comfortable for me, so I like to take it when I do longer rides. This system is great because of the flexiblity - one minute I've clipped it off to ride in a tri, the next I've clipped it back on for a group ride...then with just another clip, I can put the larger bag on it to carry extra water for those hot rides out in the country with no support. Plus, the little clip thing can be mounted on multiple bikes so that if you just want to switch your bag from one to the next, it's super easy.

I also like that is has NO velcro anywhere after I lost a pair of brand new shorts to the dreaded velcro rub on a different seat bag.

For heavier loads though, a rack would probably be better.