while i haven't flown an rv-10 yet,
i had considerable experience in a DA-40 TDI (first generation thielert) with a G1000 glass cockpit, which was also the last certified aircraft i regularly flew before transitioning to our RV-7A.
as others have noted, the stick is a non issue, it's so much more natural and you'll never want to go back. in fact, only a sidestick ? la long-ez (about the single best aspect of that airplane) would be better yet.
what i found valuable about the DA40 experience in retrospect was that it teaches you to fly gently and smoothly. the stick feels really precise, especially in the pitch axis, but it's not as well balanced as in the rv's. also, the tdi is severely underpowered during landing/takeoff, which again teaches you more precise handling. cruise performance was fine due to the turbo, though. think of learning to drive on a small car with a tiny engine. just not as forgiving as a powerful automatic. pipers and cessnas are trucks, gear takes a lot of beating etc... in that sense the diamonds are likely as close as you can get to the general handling of an RV in the certified world (less aerobatics).
payload/to performance sucked, that's where cessnas clearly have an edge. it was essentially a two persons plus baggage airplane.
if you can arrange flying a da40 (or da20 as a matter of fact) instead of cessnas and pipers during the build, great, do it! however, i wouldn't spend money on da40 time just to transition to the RV. rather spend that money on extended / good transition training. it's still plenty different.
i guess i'd go as far as saying "flying anything as much as possible during the build" does it. too many builders i know cut back too much on the flying during construction, which steepened the curve once it came time for flight test...
i'd opt on the side of taking half a year longer to complete but still be able to fly regularly.
if you have more questions re the da40, feel free!
regards,
bernie