I've ridden many road miles and done lots of group rides. I can only recall one chain failure - and it wasn't mine. Still, if you want to be prepared for all eventualities (and be popular in the group) then having a chain tool along for the ride makes sense.

I have a CT-5. It is a well made little item that could probably be your only chain tool if you don't do much wrenching. The bigger, badder shop version (I have that one, too) is much easier on the digits during use, though. And I'm sure I'll be passing it down to my posterity as it is simple and over-built.

The chain tools that come as part of multi-tools are generally more frustrating to use and take more effort in the process. If you can stand a few extra ounces and have the room in your seat bag, the CT-5 would be easier to use when you're already in a dark mood...