I got a Blackburn Flea front + rear set today. Here is my review:
The charging system is neat. There is a little piece that magnets to the back of each light and has wires that magnet to a standard 1.5v battery to charge the light. It is not any more efficient than using batteries in the light, but is easier.
I plan on rigging something up to charge it via an AC/DC converter or maybe via USB. I will report on that later.
The front light is easy to mount to the handlebar with the supplied strap. It is easy to adjust the light direction up or down, but not left or right. The light has three on modes - dim, bright, and flash. Dim is dimmer than my Blackburn Voyager 3, bright is a bit brighter. Flash mode is a bit brighter than the flash mode on the Voyager 3. The Voyager 3 has a few advantages - easy left/right adjustment, and easy removal for use as a flashlight.
The rear light has a strap as well. The obvious place to attach it to is the seat post. This unfortunately points the light at the ground as most seat posts are angled back. The strap is not long enough to strap over a reflector that is already present and pointed in the correct direction. The strap is too long to take the reflector off the reflector mount and strap the light to the mount. I ended up attaching a spare reflector mount to my seat post and strapping the light to the reflector mount with the strap going around the front (LED side) of the light, just below the LEDs. I am not happy with the setup, but I guess it will work. The rear light is a bit dimmer than my Blackburn Mars 3, more difficult to adjust, has less horizontal spread, and lacks the side facing amber lights.
Conclusion: Get the Voyager/Mars 3 combo and deal with changing the batteries.
The major downside of my original mounting of the rear light is that it blocks most of the light otherwise visible from the side. So I taped a piece of styrofoam to the seat post so I could wrap the strap around the seat post and get the right angle. It seemed like a good idea until I saw my reflection on a glass building and realized that with the light so close to the seat post, my legs block the light from the side anyway. My state requires red or amber light visible from the side at night, so this setup is not even legal. The mounting kit for the Mars 3 puts the light far enough back that the amber light is visible from the side. I think I will replace my broken Mars 3 and keep the flea rear as a backup.
Another downside - when the battery gets low it does not dim, it shuts off. I would rather have a dim light than no light. I ended up attaching the rear light to my helmet, which was easy enough. The recharging system is pretty nice - I can charge the light without taking it off of my helmet!