I have the Park I-beam Mini, and I've only used it on others' bikes. But I also agree with those who carry a few Allens and a small screwdriver. I'd suggest a spoke wrench too.
I decided to start carrying a chain break (and spare link) when I got a nine speed, after hearing they're more likely to break. My day trips tend to be in remote places in mountainous terrain with no cell service.
I've met people carrying tools who have no idea how to use them.