Like a couple of you have mentioned, it's not waterproof. If I put fenders on the bike, though, it'd be pretty safe. There's not much rain that could hit it from the top, as it's hidden under the seat, and with fenders it wouldn't get much splashed up from underneath, either. Also, there's not muchthat's exposed to the elements that could damage anything. Even if it did get wet, all you'd have to do to fix it would be dry it off. It's not really cmplex enough for a few drops of water to be able to break anything.

I live in Northern California where it's pretty dry, though, so I haven't worried about it. Maybe I'll make a more waterproof version eventually.

The batteries are pretty easy to replace. Only slightly harder than a standard taillight. It only takes a minute.

I also hacked off the ends of the screws with a dremel, like decline suggested, and it does look better.

I'm actually thinknig of building one completely from scratch and using an 8051 microcontroller so that I can do my own blinking patterns, including the knight rider effect. I'd have to borrow some equipment for that, though.