I'm in love with the superflash so take my review with a grain of salt.

How effective has it been for you? Very effective. I think it's most noticable in the blinky setting, but even on solid, it's pretty damn bright. I let the BF borrow mine before he got one and some cager actually gave him a compliment on how bright it was while he was riding at night.

How well does it hold it up? Seems to be holding up pretty well. I've dropped one a couple of times on the ground (I had one of them clipped to a less than ideal spot on my messenger bag) and that one still works.

Does it get affected by rain? It doesn't rain that much in LA and I haven't ridden in rain yet, so no idea here.

How often would I need to change the batteries? Of the two I've gotten two months ago, I haven't had to change either. I think they could go for another month or two.

Where and/or how many places could it be mounted? It comes with some mounting material. I have one mounted on my seatpost and one I clip either on my pants or my bag when I ride.
For the $20-25 I paid for both (got a discount on the first one, paid full for the 2nd), I say they're the best in the price range. The BF doubted the superflash's greatness and got some other branded rear blinky, but he soon saw the light and ended up getting a superflash for himself.

One thing that I like about it, but is pretty minor, is that there isn't a pesky small switch or button to have to look for. You just squeeze the bottom part and the light goes from blinky to solid to off. It comes in handy when you're already on your bike and you need to quickly turn the light on without having to pay too much attention to it.