I'm guessing you already decided yes or now, but I'll add my two cents anyway!

I have a bike locker at the BART station (train) and I love it.

First off (and this doesn't really apply in your situation), I had a bike stolen from the station right in front of the Station Agent's booth, right at the main entrance. My friend had 3 bikes stolen at the same station. When I spoke to the BART police about my stolen bike he said that he had never heard of one stolen from the lockers (in his 15 years).

The thing I really like about the locker (besides the security) is that I can ride up, put my bike in the locker and get on the train. No removing the headlight, taillight, battery and computer. I also keep an extra pannier on the bike with tools, extra tubes and rain gear - no need to remove it. I've considered keeping some gear in the locker, but I don't see the need since I can keep everything on the bike (and the extra weight is good for me since I have a short commute).

$15 does seem a little high, but in my case I would pay it - I'd be devastated if I had this bike stolen (even though it's another used old steel bike I love it). In my case, the locker is $30 a year - easy decision for me!