POSITIVES
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It is too early to comment on reliability, but the manufacturing seems to be of high quality.
Benefits over RF-602 trigger: FEC adjustable, HSS, E-TTL II capability. Has a smooth-acting clamping ring and pin to secure the foot in-shoe, and both TX and RX have on/off switches.
65m to 80m between camera and remote flash.
Will work with only one flash - a second flash is not required for remote control.
Frees the photographer from limitations of Canon's wireless communication - any angle from remote, no line-of-site, greater distance, not affected by sunlight, no need to orient body of remote flash.
A possible, but limited, substitute for either the OC-E2/3 TTL cable or the ST-E2 controller.
Focus assist light and modelling light available.
Quick and simple to set up.
Safe - no cables to trip people or crash stands.
Allows flexibility with lightboxes and other modifiers.
Can be used on a flash bracket without the drag of a coiled OC-E3 cable, and can be taken off-bracket in moments.
NEGATIVES
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As the camera's shoe is the only way to connect a Knight transmitter, a Speedlite cannot be used on the accessory shoe for on-axis fill. However, a Manual-control speedlite could be held by bracket, stand or human.
Lose access to effective distance indicator/flash output.
Lose 6-stop FEC unless on a 1D3.
A backup TX and RX are moderately expensive.
Need to remove shoe-flash from RX to change Group and Channel selection, and to switch RX off. It will be overlooked -flat batteries!
RX LED indicator and LCD display of selected Group and Channel hidden by Speedlite body.
Need to buy more Mark II flashes for remote power setting capability.
No E-TTL Master/slave, ratios, groups.
Cannot control flash from camera Flash Control menus.
RX shoe not designed to accommodate weatherproofing sock of Mark II Speedlites.
It does not fully replace an OC-E2/3 cord, especially as with a "Flash Control" camera, you can control full master-slave groups, ratios and power levels from the camera.
It does not fully replace an ST-E2 controller for similar reasons.