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I have 3 flex's, 2 mini's and an ac3. For most things I just use the flex as a master with the ac3, and control two lights. That way I can avoid using the mini and it's button battery.
I will use the mini (and I have a g-wiz bag with a spare button battery just in case), when I have to control three flashes, or sometimes when it's on a camera that has a function dial on the left (D300, for example) which can be blocked by the flex.
button batteries are cheap, so I'd buy a few to have handy if possible. But when I got it, the original owner told me their first one lasted a year or so, and never showed low battery.