I have a few Bikes so I ordered both the Fast Flash and XP-60 simply because they do not explain the difference in the two, so I will do it here.
The XP-60 will allow your break light or any light to flash when activated. The Flash sequence is slightly different or seems to be slightly different than the Fast Flash. The XP-60 has 1 quick flash followed by 3 quick Blinks, followed by 3 slightly longer blinks then a nice steady on. The XP-60 is in no way a frantic flasher, it is noticeable and will get the attention of the people behind you, just not startle them. The Fast Flash, is just That a FAST very Fast Flash. Activate and it is very Rapid (2-3 times faster then the XP-60) 3 burst, then 3 flashes much like the XP-60 first the flashes, then it Stays a strong steady break light. The Fast Flash is a Frantic attention getting flasher!!!! It will wake people up behind you!!! It will annoy them for sure if they are following you in steady traffic.
Installing them was very easy, Follow the fire harness from your tail light to where it connects into the main wire harness, you will be able to unplug your tail light there. You should have 3 wires, power, ground and break light. Power is for the constant on part of the tail light, the break wire is the one your looking for as well as the ground. The Flasher unit is clearly marked In and out on both sides and has 2 wires on both sides. Hook the (+) to the power wire and the (-) to the ground. Turn on the key to the bike and your light should look normal. Squeeze the break and you should get the flashing sequence before it stays a steady on bright break light. I add two small LED marker lights below my plate, so they will flash in and stay steady on while my break light will operate normally. So you can add up to 60 Watts of lights with these little boxes, or a bunch led lights (because they use almost no power) The whole wiring process is super simple and will take just a few minutes if you are just using your break light. If you are adding lights like me, then of course it will take a bit more time. They do supply you with some really strong double back tape to mount this tiny box where ever you have room. The box size is very small, maybe 1/2 to 1/3 credit card and 1/3 inch thick. it is smaller than it looks, so you will have plenty of places to locate it, even on race bikes. They work well, I will post a couple of videos when I get them both hooked up on the bikes. Again I added two marker LED's to my bikes, which I purchased from Superbright LED.com. They also sell here on Amazon.

Easy to install. Had to do a little test to figure out which wire was the brake vs running lights on my Suzuki M50. To test your connections just use a multimeter or pop the pins out of the connector and systematically test them. Wrap the wire from the fastflash around the pin and test. Resulting light is a little brighter (60W maybe) and the blink pattern is much more noticeable than just a solid red brake light. Very happy with this mod.

So far a great product. Got to my door quickly and well packaged. Even came in an anti static bag which was cool, if not a bit overkill. The instructions were very easy to follow, and from the time I opened the garage to having put the last clip back in the liner for the trunk it took me all of 20 minutes to install. For the price and the time it took, I say its well worth it to add even a little bit of safety when you drive on the freeway as much as I do. Did not dim the light at all from the install. The only con I can think of is that it did not come with any connectors of any kind. I have my own weatherproof connectors lying around, so that wasn't an issue for me. Hopefully it lasts a good long while.

Initially got FF 24 for my single brake light on my motorcycle. Had some issues but the customer service is amazing. Helped me end up with FF 60 which makes brake light brighter overall with addition of flashing.

Btw...installed on Honda Interstate VT1300 CT with red LED from Amazon.

My bike had three wires. Green/yellow. Tan. Green. Connected black input from FF unit to Green (ground). The green/yellow was cut and the bike out went into red in on unit and the red out to Green/yellow heading to bulb.

Helps others stop! I have a 2 year old daughter and it seems to back people of a little faster. It gives me a little more comfort driving knowing that this will grab the attention of drivers. Especially the ones with their faces embedded in cell phones! Not to mention the added head turns when you hit the brakes. Great product.