I have a set of KCNC and haven't been to impressed. There is visible flex and with shimano pads, power is OK even with only 130lbs on them. I ordered a set of SwissStop Greens, but I'm thinking abou trying something different.

Brake vs. Brake will I see a difference between the two?

Are the single pivot Planet-X's really that fiddly with centering?

I've used both. The Planet X are superior. Stopping power is about the same but modulation is much better with the Planet X as there is less arm flex. The difference is most noticeable under heavy braking when arm flex occurs. Aesthetically I like the Planet X better. The KCNC develop a lot of friction in the arms when they get dirty. Maybe the quick release is a little better on the KCNC. Finally, I've never had any problem with centering. The design of the Planet X causes one of the arms to contact the rim before the other when braking. IOW the pads do not approach the rim simultaneously...this is of no functional consequence.

Seems to be mixed reviews regarding PX brake centering.
My experience was too fiddly, they may be fine for rolling hills & flats but on long steep decents (and I expect in crits) the confidence in them was not there.
I moved to C7s (dual pivot).
Definately get some good pads such as SwissStop.
If you are still uncomfortable you could consider slightly heavier but benchmark performance brakes such as EE or Shimano.

Just wondering... any love for Far and Nears? More expensive than PX and weigh a tad more, but nice looking and standard dual pivot performance.

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formerly rruff...

I have had both as well . If you know what you are doing the PX brakes are easy to set up and easy to clean and look after . Kcnc brakes are a bit lighter but to clean them properly you have to take them apart which gets a pain in the ar%e . I put some kcnc pad holders on my PX brakes which makes them a bit lighter . As for breaking the PX are much better they stop really well and def the best stoppers of the lightweight market. I have tried most of them although I have not tried gravitas yet . If you do get KCNC I would go with gooey pads, they helped with the stopping a lot, they work really well but wear down a bit quicker in the rain .

Never owned Planet X brakes but the old Frm are kinda similar design and they were just ok; Id compare them to the old CB1.
The C7 which I just purchased are slightly better than others and yes the pads might be a bigger factor than the brakes itself.

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ciclismo-espresso.com

+1 on C7's being a total pain in the arse! Constant cleaning unless riding in absolute perfect weather, with perfectly clean roads. And, on top of that, engineering wise, they are just the utmost definition of complete chinese crap! There are no bearings (nylon or otherwise) in the pivots, so they seize up and become gritty pretty much from just dust...

And the flex, it would have cost less than 5g of well placed material to reduce atleast 50% of the flex... it's just a plain stupid design, cutting the most weight where it's needed the most. The whole arm doesn't just flex, it even twists, and on top of that - the arms rub each other when they flex under braking load, adding even more friction...

Sure, the stopping power is decent for the weight with good pads. They work as brakes. But what gets to me, is just the absolute stupidity of the design, it's nothing short of horrible engineering! And the finish of the product screams of cheap chinese crap...

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If you are at all mechanically adept it's no hassle at all to center the Planet-X brake. Set it and forget it. I've put thousands of miles on them since buying them this Spring and haven't had to touch them once. As for stopping power, if I can grab the levers and practically launch over the bars if I squeeze too hard how much more stopping power does one need? Got them on sale for $125 USD. Pretty and under 200g with pads. Swiss stop yellow w/ Easton EC 90 SL tubulars.

My very mechanically adept (an engineer) so sounds like I shouldn't have a problem setting the Planet-X brakes up. I agree with the previous posters complaint about cleaning the KCNC, they're a total PITA. You have to disassemble to the whole brake. Sounds like the x's are worth a try. I have two bikes and will do a head to head comparison.

Not to single you out but as a bike mechanic I've heard those famous last words many times...

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considering i've restored motorcycles and rebuilt cars... I think i'll be ok.