I have the S-12. Bought it in the spring to use for TransRockies. Prior to that I used the Nathan Women's intensity. I had used the intensity for 6+ years of ultra running and had no issues with it for my races (up to 100). I upgraded to the s-12 mainly because TransRockies has a required gear list and I wasn't sure that I could get it all to comfortably fit in the Nathan. Overall I really like the s-12. It's very comfortable and doesn't bounce. It has great gear storage and storage in all of the pockets is functional and comfortable. I will have to say it is a bit "hotter" than the Nathan but not enough to make me not want to wear it.

I have the small and I'm 5'2 (in case this info helps). Let me know if you have any specific questions and I'll do my best to answer.

Mostly I am wondering about sizing- the xs/x is supposed to fit chest sizes of 36 inches or less and the m/l is for 37-42 inches.

being a girl, my chest is different sizes in different places: with a tight running bra it's about 38 inches around my boobs and 34 just below, so which measurement do I go with? (I'm 5'10, if it matters)

Do you have any issues with chafing? I worry that the pockets on the front will scrape the insides of my upper arms.

And of course there's the price...I'm trying to justify that it's about what I'd spend on running shoes in a few months...is it really worth it?

I haven't had enough coffee to clearly articulate other questions now, but if you have any other advice I'd love to hear it.

Thanks!

-L