I have the Garmin footpod. I got it when I started running because everyone I asked (who knew what they were talking about) mentioned the importance of a high cadence. Keeping it up was much harder than I expected 🙂 but I gradually built up to 90 per foot.
As said above, once you know what it feels like you don’t really need it, although I suppose it’s interesting from a stats point of view, you can see how tiredness/hills/etc, effects your cadence. Also as mentioned it allows you to collect data when used indoors or on a treadmill, personally I’ve never used it like that though.
I also have a clip-on digital metronome as pictured above, basically allows you to do the same thing. It’s a lot cheaper but you get funny looks sometimes when you’re ticking loudly as you run past people!