In March of 2000 I purchased an FL318 taillight for my bicycle at a local Target store. This light is advertised as a Safety Flasher, mostly for use with automobiles. It has an array of 18 LEDs that operate off of 4 AA batteries. It can operate either in steady on mode, or flashing mode with a flash-rate of about one flash per second. It has two rare earth magnets on the back that allow it to adhere to any metallic surface (such as the side of a car). At the time this light cost me about $12, which is about the same price that it is available for from Cutting Edge now (once you add shipping).
I didn't trust the magnets to adequately hold the light on my bike while commuting, so I needed to come up with an alternate mounting system. I accomplished this by drilling through the body and the battery cover, then running a screw through so that it sticks out the back. The light is then mounted to a bracket on my bike with this screw and a wing nut. Using a wing nut allows me to easily take the light off for battery replacement.
Shortly after I purchased this light, I heard about the RealLITE from Necessary Options. At the time I thought that the RealLITE was identical to the FL318 light that I had with the exception that the RealLITE included a mounting bracket for bicycles. However, Necessary Options claimed that the RealLITE was a much better, brighter light. Since the FL318 seemed to be bright enough for me, and since I had already come up with my own mounting bracket for it, I decided to just stay with what I already had.
Three years later, my FL318 started to act up (wouldn't stay turned on), so I went ahead and ordered a RealLITE from Harris Cyclery. With shipping, this light cost me $48. When it arrived, I compared it to the FL318. This page documents what I noticed from this comparison. Keep in mind that my FL318 is three years old, while the RealLITE is new.
These photos show the two lights. In all cases the RealLITE is on the left, while the old FL318 is on the right. Click on the image for a larger view.
Quite frankly, it appears to me that these lights are identical with only small exceptions. The RealLITE comes with a more secure mounting system, while the FL318 will require the user to come up with his own mount. However, I believe that the RealLITE's mount will make it awkward to replace the batteries.
The flash rate of the two lights is also worth considering. My old FL318 has a rate of about 57 flashes per minute, while the RealLITE flashes at about 72 per minute. I've always felt that the FL318 is too slow, so if I am going to run it in flashing mode instead of steady on I usually use an additional Vista light along with it. While the difference isn't all that much, the flash rate of the RealLITE feels better to me, so I would be willing to run it alone.
At the beginning of this I mentioned that my FL318 had started to act up. Basically, I would turn it on when I left home, and on arriving at my destination I would find that it was turned off. I originally thought that the switch was going bad. However, while disassembling my light for this comparison I found that one of the screws that connects the battery compartment to the circuit board had come loose, creating a flaky connection. Since tightening this screw down, my light again seems to be functioning properly. I had hoped that the RealLITE would have a better switch. However, both lights use the same switch system which appears to be a set of traces on the circuit board which detect when a magnet in the push-button switch comes close. This was probably done to keep the unit water-resistant, and mostly seems to work, but I can easily see how the rubber in the push-button will break down over time and allow the magnet to bounce more, causing false button presses. I would prefer a more positive connection switch, but for the most part it seems to work. For now I've gone back to using my old FL318 and I'll keep the RealLITE on the shelf for if the FL318 does decide to completely die.
In the final analysis, then, is the RealLITE worth three times as much as the FL318? That is a question that you will have to answer for yourself. If you want a light with a good mounting system and the higher flash rate that you can just throw on your bike and go, then the RealLITE is the way to go. If you can work up your own mounting system and you are comfortable with the slower flash rate, then the FL318 can save you some money.