If you find yourself regularly riding at dawn, dusk or in the dark (commuting, say, or bar hopping), you really want your lights to be bright. Really freakin' bright. Here in New Mexico, where drivers are known for their obliviousness, I do my damnedest to be lit up like a 'Vegas Christmas tree for my morning and evening commutes. Starting last winter, I started seeing a few freakishly bright tail lights out on the road. It turns out that they were Planet Bike's (then) new Blinky Superflash.
Using one massively bright .5W LED (in the white section of the light) and two smaller LEDs (behind the red plastic), the Superflash has a distinctive blink-blink-BANG flash pattern. This thing puts out more light than anything I've seen this side of DiNotte's $160 140L-AA-R rechargable tail light. Eventually, I tracked one down at the local hipster bike shop and sprung for it.
Like all LEDs, the Blinky Superflash's output is very directional- the closer you are to viewing it head on, the brighter it gets, making proper mounting (facing straight back, parallel to the ground) essential. From head-on, the Blinky Superflash is, basically, unviewable. The primary LED puts out a massive amount of light, making the other two (standard brightness) LEDs look quite feeble by comparison. Planet Bike claim that it's visible from a mile away, and I don't doubt it.
Last fall, I made Blackburn's Mars 3.0 tail light a bikefix Pick for its reasonable price, and 180 degree visibility. From head on, the Superflash just puts it to shame. The balance on backpack blinky tabs is quite good, with more weight below the clip than above (a good thing). Because there are no side-facing LEDs, though, any side visibility is coming from the main LED- similar to the Mars (but better than I expected), with a good deal of light headed skyward. The unusual flashing pattern may or may not be scientifically designed to grab drivers' attention better than the standard strobe- I could see it working either way. I suspect that it might be a battery conservation measure (that big guy must take a fair amount of juice). I'd almost rather the secondary LEDs be pointed off to the sides a bit, increasing the range of visibility without compromising rearward visibility.
For the ride to and from work, I opted to leave a Mars 3.0 on my each of my commuters' seatposts and mount the Blinky Superflash to my helmet (to further set it apart from other drivetime tail lights). With careful positioning, it's visible to drivers and sits in my locker all day, all tosty and dry. So far, so good. I've had comments from coworkers on it's brightness and I feel just a bit more secure with it up there. At $25-30 in shops, it's $10 more than the Mars, but considerably brighter. The advertised battery life (on 2 included AAA batteries) is 100 hours (flashing)- twice that of the Mars. It's not cheap, but if it saves you one ER copay, it's more than paid for itself. Oh, and Planet Bike donate 25% of their profits toward bicycle advocacy. I'll run it all winter and let you know if anything changes. So far, though, it's hard not to recommend.