I have used many LED tail lights over the years since I do more than share of night riding on brevets or double-century rides. Here is quick review.
Recently I purchased the 2-watt Cygolite Hotshot because it has a USB rechargeable battery, which I was wanted to use on my daily training rides (I put a piece of gaffer tape on the top of it to act as an awning so it shows, even during the day). The Hoshot mounting system is a really bad design. Despite tightening to the maximum, it still drooped as shown in the photo, pointing down to the ground. So I taped it on the top, holding it parallel to the ground and then on a 50-mile ride, it just fell off (unbeknownst to me!). This light always had too many adjustable settings and was confusing. Although I like their Turbo 740Xtra headlights a lot, I cannot recommend this tail light.
The PrincetonTec Swerve has a great 1/2 watt lighting pattern (I don’t recommend anything less than 1/2 watt), but it also droops a little too (but not near as much as the Cygloite Hotshot).
The Planet Bike tail lights have reported leakage problems in heavy rain, so many cyclists mount them upside down. I have never used them and have found the lighting pattern on the Swerve and Tail Blazer to be superior anyway.
Finally, the 1/2 watt Sigma Tail Blazer is now my (Feb 2013) “go to” tail light for night use and long rides. It doesn’t droop and the mount is very secure (although it is slower to mount the bracket than others initially, so I would recommend separate brackets for each bike). This spring they are releasing a USB rechargeable model called the Stereo which looks promising. It has two 1/2 watt LEDs.
This summer (2013) I purchased the new Sigma Stereo. It is a USB rechargeable tail light. I used it, simultaneously with a Tail Blazer this summer while on a 3-day 1200K. The Tail Blazer was hands down brighter, according to other cyclists that rode behind me. The Stereo wastes it wattage by illuminating a cute ring or border around the perimeter of the light, which does nothing to attract motorists at a distance. I cannot recommend this tail light either.
I have seen many lights that are have less than 1/2-watt output. Typically, these lights have three or more weak LEDs and although the multiple LEDs may look cute in the store, they simply don’t have the punch or visibility to motorists. I remember on a 400k ride, a friend of mine had one those off-brand 5 LED tail lights. It looked very dim. I mentioned it to him and he changed out the batteries, but to no avail. I asked him what brand it was. He wasn’t sure, but that he got it on eBay for $3 or $4. Later that season he got bumped by the mirror of a passing motorist while commuting home on his bicycle! Fortunately he was not badly injured — in fact he stayed upright and was not knocked over. Shortly after that, myself and another friend each bought him a 1/2-watt tail light.
Your safety is not worth cutting corners when it comes to lighting!
November 2013 Update: I picked up a Lezyne Micro Driver Rear. It is designed ONLY to fit on a seatpost, but I typically have a saddlebag there, so I adapted it (hot glued some foam on the connector) and mounted it upside down on my seat stays. I love this light for daylight use! It is USB rechargeable and is incredibly bright. I also made a small awning out of gaffer tape, so the light is more visible during the day. I now use it regularly on training rides. The build quality (CNC machining) on all Lezyne products is incredible.