Before this, I carried around a set of folding hex keys. That worked well enough, but was a little heavy, so I replaced it with this little guy. Fits all the stuff I need to get to in a pinch, although the hex keys on the sides can be annoying in a tight spot (say, brake shoes right next to the seatstays and fenders) because the body of the tool gets in the way.
I carry this when I'm racing triathlon, for peace of mind and to keep Murphy away. Small and light, it goes along with my flat repair kit. I also carry it when I'm cycle touring, along with a larger multi tool. When you need to do serious repairs on the road, sometimes you need tools in each hand.
After looking at several multi-tools and their reviews, I decided on this one because of it's simplicity. There is only one piece to the whole thing so I don't have to worry about anything coming apart or loose. Plus, I can quickly see which head I might want to use without any opening and closing.
I've owned a few mini tools in my day and bought this one after one of my friends showed me his. What makes this better than any other mini tool is it's LIGHT WEIGHT! It fits better in back jersey pockets too. Doesn't have as many tool options but it gets most any job done when you're on your own out there.