Well, now I can answer my original question. I've got about 100 miles on my new Checkpoint. I've ridden it on a variety of surfaces, but nothing too challenging at this point.
My initial impressions are mostly very positive. The frame geometry (adjusted to a 435mm chain stay) provides handling that is exactly what I was looking for. The frame is solid and consistent. Very controllable on a variety of surfaces. The weight is very good and the feature set definitely makes this a "Swiss Army Knife" of a bike. Extremely raceable and extremely versatile at the same time. It has the handling of my older Niner RLT9 (which I considered just about perfect) without the harshness of the aluminum frame. It feels lighter and more nimble than my Lynskey custom (today's stock Lynskey geometry).
Two reservations: (1) The BB90 bottom bracket means you're restricted from using the few modern sub-compact chainsets on the market. (Why won't somebody make a modern sub-compact with a BB90-freindly 24mm spindle? FSA . . . I'm talking to you!) and (2) I'm not completely sold on the Isospeed decoupler. With it, the seatpost flexes visibly with every pedal stroke -- and that's got to be inefficient, if only slightly. Certainly less efficient than a solid seatpost would be. You cannot discount the comfort advantages of this system, but I seriously wonder if the loss of efficiency is worth it.
The bottom line: I like the Checkpoint SL a lot. How could it be made perfect? Give it a T47 bottom bracket. And . . . I'm not so sure about the Isospeed. Make it adjustable (as on at least one of the other bikes)? I would probably prefer that the bike had a standard seat tube so that I could experiment with seatpost flexibility. As it is, Isospeed locks you into a springy ride. The Checkpoint SL is a keeper. I can see myself riding this bike for a very long time. It's just not quite perfect (in my eyes).
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Last edited by FlashBazbo; 10-01-18 at 10:08 AM.