My quick answer is that once you are used to any shoe that is zero to 4 mm ramp (aside from Vibrams with no padding), any padded low ramp shoe has zero adjustment time other than the break in time for a pair of running shoes.
Personally, if I was doing IMFrance, I’d stick to the Newton at this point, because you know it works. I’ve only done in a few runs in the ON and I would say it would work fine.
Actually while running in the ON’s I had some addtional thoughts that I did not put in my summary. The ON’s are “soft on initial impact” and hard on “closure”. The Newtons are the opposite. They are harder on initial impact and get soft thereafter.
My best analogy is how we describe XC skis…stiff initial camber, softer final camber (Rossi’s) , versus soft initial camber, hard final camber (Fischer). The ON padding mechanism gives easily at first until the compression chambers “close” and then it is first. The Newtons on the other hand are stiffer at first but softer as the lugs compress. In a sense the Newton’s feel more springy, while the ON’s feel more forgiving especially at “slow speed”.
Maybe another way of describing it is that the Newton seems to be more responsive, and the ON more cushy. I can see the ON actually being a nicer shoe at slower speeds while the Newton being a better shoe at higher speeds where the impact force is higher.
That is just my personal opinion, looking at the mechanism and running in both so far, but I don’t have enough experience in the ON.
I went to the track with my ON’s and Vibrams and was running 100m accelerations in my Vibrams at my standard speeds and then did some 400’s in the ONs and SEEMED to be 1s per 400m slower than I was 2 weeks earlier in Newtons. In between the 2, I ran the St. Croix half IM, so it could be St. Croix in my legs, but then I SHOULD have been running slower in my 100m accelerations in the Vibrams…then again, 100m is not enough for fatigue to kick in (given that I was recovering from a half IM). So I need to give the ONs some more rounds at the track. They did seem to run “heavier” and “duller” than the Newtons for 400m repeats, but as I said, this could just be “me” being tired and nothing to do with the shoe. I did get kind of sick on Sunday evening, so that may have been it.
So given that I have had 5 years in Newtons and 5 days in ONs let’s wait for some more time to give them a fair shake. So far, I am actually quite impressed with them and thinking about using them for my next Ironman…and my left knee that is banged up from an accident in 2011 really seems to like them better given that the initial impact is “soft”, but there is more “stability” at final closure and when I am doing an ironman my pace is so slow that this might just be perfect. At a 5k or 10K race, no, I would not use the Cloudracer. I probably would not use them in an Olympic tri. Maybe a half IM. I don’t see the Cloudracer as much of a racing shoe. I’d classify it in the lightweight trainer category.