Recently I purchased a pair of "Luxeon" 3 watt LED bulb replacements for my motorcycle, a 2006 Kawasaki Concours. The tail lights on my motorcycle are the combo running-light / brake-light that many tail lights are, and use bulb type 1157.
There are many reviews of standard LED tail light replacement bulbs, and most of them say that they are just not bright enough. The consensus seems to be that you really need to have LED systems designed for the light from the ground up - as the bulb replacement units simply fall short.
The main problem, is that LED lights are very directional, they have a very narrow beam of light. Most tail light enclosures are designed for a regular incandescent bulb which puts light out in all directions. Because of this they use mirrors and lenses to direct the light out of the enclosure. This is the way that tail lights appear to fill up with light when they only have one small bulb in them.
LED bulb replacements typically will just shoot light out in one direction, so they do not fill the light or create a good amount of visibility from the sides.
There are new LEDs that have come on the market in the last year or so that are called "Luxeon" or sometimes "Lux". They are very very bright. In fact, in certain applications they are bright enough to damage your eyesight. However, they require more complex circuitry and usually very good heat sinks. They also supposedly have (for LEDs) a very wide beam pattern. 120 to 130 degrees.
I found some "Luxeon" 1157 bulb replacements, and decided to try them. I bought them from SuperBright LEDs. I purchased the red 3 Watt Luxeon 1156/1157 bulb for $24.99 each. This is their description at the time of writing this:
These are by far the brightest LED Car Tail Light bulbs on the marketSuperBright LEDs shipped the order VERY FAST, and it came exactly as ordered. The Kawasaki Concours has two bulbs together in one tail light, so it is very easy to compare side by side with the stock bulb. The stock bulb is the bulb that came with the Concours, it is not a high power bulb or anything custom. It has been in use for about 3000 city miles, just under one year.
Our exclusive LX3 series bulbs use High Power 3 Watt Luxeon LEDs to achieve light outputs even higher than standard filament bulbs
3 Watt LEDs feature super wide 130 degree beam angle to illuminate entire tail light lens
Here are the pictures so you can decide for yourself. I tried to duplicate conditions for each picture as best I could, it was in the evening and in February in Portland the days are short, so my light was running out quickly.
The Luxeon LED is on the left, the stock bulb on the right. In all pictures the motorcycle was running a high idle with adequate voltage getting to the lights.
So, I decided that the Luxeon lights were just not right for this application. They may or may not work in your tail light configuration, as a lot depends on the mirrors and lenses. I returned the lights to SuperBright LEDs, and they were completely awesome. I highly recommend SuperBright LEDs for your LED needs, and I will definately be watching them into the future as more and more LED products come into the market.
I also might try the Eagle-Eye Tower from AutoLumination as they have the bright 3W Luxeon bulb facing out, and 10 bright LEDs facing the sides to light up the mirrors. I think these might fill the tail light with light better, using more of the built in reflector. But I found in some forums that some of the lights from AutoLumination broke fairly early, possibly due to a lack of vibration resistence. A car or motorcycle is an environment with lots of vibration, so at $25 a bulb it could get expensive if the lights didn't last. I do not have any personal experience with them - it was just what I read that led me to try the SuperBright LEDs product first, as they have a good reputation amongst auto enthusiasts.
In the end, I think that currently the best bet for the Concours is to go with custom LED solutions instead of a bulb replacement. However they tend to be more expensive. For other tail light applications, you might have better luck. But a custom LED designed for your car or bike will probably always work better than a "drop-in" one size fits all attempt.
And as always, your mileage may vary.