Check out youtube videos comparing Planet Bike taillights with the Portland Design Works Radbot and Danger Zone.
Also think about the fact that if you're going to buy a MagicShine headlight, for $30 you can get their taillight and a Y adapter. If you were going to spend $20 on the Planet Bike taillight anyway, the MagicShine taillight is a lot brighter for just a bit more money.
However, if you will only be riding at NIGHT and not at night in bad conditions, one of the AAA powered taillights is sufficient and is less screwing around (the MagicShine taillight can be a little bit of a pain to mount, and you'd have to run an extension cable to it from the battery). I'd seriously consider something like the Danger Zone though instead of the PBSF.
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Work: the 8 hours that separates bike rides.