The Planet Bike Superflash and Cateye TL-LD1000 are two very interesting contenders. I have both on my bike and they have survived rain, snow... and even a few sunny days.
– The Superflash is about half the price (or less) than the LD1000. It's also smaller, so if you have a crowded environment, it's an advantage.
– The Superflash flashing mode is really attention-grabbing. Great for foggy or cloudy days, but I won't use the flashing mode at night, unless I travel on Las Vegas Casino street. By comparison, you can use the LD1000 with one row steady and one row flashing at night.
– Both can be rigged onto a rear rack with a little ingenuity. In theory, you could bolt the LD1000 on the flimsy reflector bracket that comes with most racks, but I would not recommend it unless you live in a world of perfectly smooth roads. I used an aluminium angle bracket and bolted the LD1000 and secured it with 2 zip ties; I clipped the Superflash onto the angle bracket (I saw a slot for that) and secured it with a tiny bolt. Nothing obvious, but it works.
– The LD-1000 seems waterproof so far. As for the Superflash, the results are mixed at best. I have fenders.
The LD1000 and Superflash I have on my seatpost are protected by my rear end, so even riding through a 1 to 1.5-hour downpour, no problem. On the other hand, I have a Superflash and a LD1000 behind my rear rack, so they are exposed to rain, car splashes and some midst from the wheel; the LD1000 has no problem, but the Superflash failed. It worked again once I dried it. So now the Superflash lives in a plastic bag, which makes it as useful, but less pretty.