Just my observations from having owned (and loved) both the KSO's and the KSO Treks:
1. Try not to order online if possible. Fit varies widely, and even varies between these two shoes. My two pairs are different sizes. In my experience, I also ended up with a size larger than I thought I would, (as compared to other shoes that use the same sizing system). A lot has to do with how it feels, so it's best to try on multiple pairs.
2. Their sizing is determined not by what size you normally wear, but by measuring your foot. You can't skip this step if ordering online.
3. Although I am female, the men's shoe fit better for me. If you are female with a wider foot, consider the men's style. If you are male with a narrower foot, consider the women's.
4. The kangaroo upper of the Treks wears better than the KSO upper material. It's also better if your feet sweat, (in my opinion) and is more comfortable.
5. I use the KSOs for running on pavement only (and only so I don't wear out the Treks so quickly). If you'll be hiking or running where there are even small rocks, or uneven terrain, the Treks are much better. I think the Treks are the perfect all-around, multi-purpose option.
6. A thin pair of toe socks help in cold weather.
7. Wear them around the house instead of going barefoot for a week, while you get used to the feel. Then, slowly break them in keeping the amount of time you run in them short in the beginning. Your muscles need to develop in order to properly support you. I actually wore mine to the grocery store and while running errands as well.
8. When first hiking or running in them, pay special attention to the way your foot is hitting the ground. You can't use the traditional "heel strike" gait, and in fact, it defeats the purpose and will cause considerable pain. Pay a little attention in the beginning until you get used to them. You're going to notice that your calves need time to adjust too, and they get a bigger workout when using these.
9. Give them a brushing with a suede brush to remove any loose dirt. You'll want to wash them if they start getting stinky, and you can, in a washing machine in cold water with a mild detergent, but don't put them in a dryer. Just air dry them.
10. Beware of the websites that sell these "really cheap". They are cheap foreign knockoffs and a friend found out the hard way that they are nothing like the real thing, (and then he couldn't return them).