In early games, you had to calibrate the remote every 10 minutes. I think it has improved via the games that use it (Wii Sports Resort was trying too hard, I think. Compared to Skyward Sword, the motion required to detect motion was highly subdued.
I wouldn't say it's true 1:1, since sometimes what it thinks is center can shift, and after playing a long session of Skyward Sword without recalibration, what started out playing on a plane parallel to the TV almost was perpendicular. But everything still worked, and I was able to find a more comfortable postition on the couch, so I left it alone. It still is a lot better than without Motion Plus (again, in the games that support it. If a game doesn't support Motion Plus, then having Motion Plus in your remote doesn't gain you a thing).