Yes, download LiveBPM to your smart phone. I use it when ever I play with musicians I have never played with before. Especially at rehearsals.
It helps me maintain the tempo. Or at least I know that the tempo has changed a bit.
Don't stress out about a 2 or 3 Beats Per Minute variation at like 80 BPM.
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Agreed--playing to a click is one thing, but measuring your tempo when you're NOT using a click is a whole different story.
It's great when someone asks whether the tempo changed during a song, and I can point to the LiveBPM graph and show them that it did not (significantly) change. (Or, if it's a bad night, it proves to me that I was indeed the problem. Good for keeping my own ego in check...)