Pros: Easy to set up -- just slide on a flash and you're done. There are no batteries, no cables, no switches, no ANYTHING. It is amazingly simple.

Cons: As others have commented, it is sensitive enough to detect the pre-flash that most modern cameras use. Thus, it triggers on the pre-flash, and misses the actual exposure. There are things you can do to adjust for this, but that limits the usefullness quite a bit.

Other Thoughts:

Bottomline: It is a good addition to all those extra flashes you have lying around, but you'll probably find it too limiting for any serious work.

Pros: Confirmed working with Nikon SB-26, SB-27 (trigger voltage of 4 volts), Black's ST 45 flashes (trigger voltage of 70 volts).
The hot shoe fits well with all above flashes.
It's quite sensitive indoors. Sensor does not have to face the triggering flash. Reflection from walls/furniture will be enough.
Both the FEL (flash exposure lock on Canon) and a -2EV main flash can trigger it.
I use canon built-in flash to trigger it. Set the FEC to -2EV.
Each shot, I have to press FEL, then hold and press the button. So two flashes will occur. This is to get around the TTL pre-flash problem.
Alternatively, you can mount a manual flash on Canon (or tape over the camera hot shoe data contacts, and put a Canon flash on as a dumb manual flash). That way, you don't have to use FEL.
It's not a good idea to use a hand to cover the built-in flash when pressing FEL. Because while it avoids the FEL triggered flashes, it causes the camera to overcompensate (because it sees inadequate light) to put out full power in the main flash. That will not only mess up your shots, it will make the built-in flash very hot and damage it if you do this nonstop.

Cons: When used on Canon 430EX, the flash will lock up after a single flash. Pressing the flash test button does not reset it. You must power off and on the flash.

Other Thoughts:

Bottomline: Best price, works as expected.

Pros: Very basic and cheap remote trigger. Works fine for my intended use. I use it together with flash SKU 11402 with good results.

Cons: Nothing really if you know what you are getting, there are no settings, can't do pre-flash but. Cheap unit so range is limited, but if you need that you should look at radio trigger units.

Other Thoughts:

Bottomline: Fun for doing experimental stuff, with the low price you can get several and trigger all your flash units. I have a setup with three "off camera" flash units and can make some fun lighting setups.

Pros: It does what it says. When it sees a flash, it fires. It fits on my tripod.
Indoors, it fires consistantly without problems.
Even though this model doesn't support pre-flashes, I have had moderate success when leaving pre-flashes enabled on my Nikon D70s.

Cons: Outdoors, if the sun can see the trigger's eye, then the chance of it working is low. Works well when the eye is shaded, though with understandably limited range.
When connected to my SB-600 flashes, the connection is very loose. It can be disconnected without releasing the hotshoe lock.
I had to tape an area of the trigger's hotshoe. Since it is loose, one pin on my SB-600 was making contact and setting off the flash when moved or bumped.

Other Thoughts: The brighter the ambient light is, the shorter the range.
The bottom can connect to a hotshoe, but has no lock to hold it in place... fairly pointless.
This one, SYK-3, is the model that doesn't support digital pre-flashes. You must set your onboard flash to manual so it does not preflash.
SYK-4 is the same as this one, except it has a sync cord option.
SYK-5 is the one that supports digital pre-flashes.

Bottomline: A good buy. For the amateur, the price definitely beats buying PocketWizards (or other radio triggers), and you get the same effect in most cases.
You don't need SYK-4 or SYK-5, this one will do, as long as you understand how to light manually.