Okay, I've spent a few hours with Wii Motion Plus now.
Basic impression: It rocks!
Physically, it adds about an inch-and-a-half to the length of the Wiimote, and comes with an extended version of the "condom" jacket that's not meant to be removed. It has a locking switch to keep it firmly in place, and a pass-through so you don't have to remove it to use the Nunchuk or the Classic Controller or any other add-ons that may come along. Changing batteries and resynching the controller is a bit of a pain now, as you have to disconnect the Motion Plus and ease the hind-end of the controller out of the jacket, threading the wrist strap partway through so it can move, but it's not much worse than dealing with the old jacket by itself.
First, the bad news: this enhancement is not the be-all and end-all of motion control. The Wiimote still doesn't have a clear picture of where it actually is in 3-D space. Programmers still have to rely on a combination of noisy sensor inputs -- the existing IR camera, angle sensors, and accelerometers, plus the new gyroscope -- and draw mathematical inferences to figure out what the player is doing. The additional gyro information is very useful, but without occasional recalibration it can get a mistaken idea of the world; to get the best precision, the system needs a reliable point of reference every so often. This is visible in Tiger Woods 10's Disc Golf mode, where you have to point at the screen to pick up your disc before every throw (the main golf game seems quite content to use gravity to calibrate your golf swing, so it's not strictly necessary.) The MotionPlus apparently recalibrates itself regularly when held still, so if you're fidgety it may not work as well as it would otherwise. So the console is still perceiving the human interface rather dimly.
The good news: The Wiimote now has a MUCH clearer picture of how it is oriented and what your hand is doing, which combined with an approximate sense of your arm's motion seems to do a pretty good job overall. The control feels like it's 1:1 -- it's not quite, obviously, as there's a little bit of lag in the process of reading and interpreting the sensors, then displaying the image, dealing with the notorious HDTV lag. And any game can impose dimensional constraints for its own purposes, inhibiting the player's freedom. But it's a damn fine illusion.
Within Tiger Woods, I can say the following: Disc Golf works just the way you'd want it to, taking your throw's angle, velocity, and release point realistically into account. Swinging at a golf ball feels great (even if the onscreen golfer has better posture and form than I actually do, and never actually MISSES the ball), and putting FINALLY works naturally in a Wii golf game. You can tap it in, you can draw and fade naturally, you can overshoot the hole, and you never feel like the Wiimote missed a step -- when you screw up, you screw up fair and square. It's the Wii golf game I've been waiting for ever since Wii Sports golf, which was fun despite its limitaitons. This game just plain works, and it feels just right.
I'm really excited to see what's coming up for this technology, because it can fix a number of scenarios where the current Wii technology is fun but clearly faking it to some degree. I would love to see Samba de Amigo redone, because while it kind of works as-is, you really have to learn how to conform to its expectations -- it's not nearly as natural as the height-sensing Dreamcast maracas were, and the Wii edition's angle-based approach would benefit greatly from MotionPlus. I would love to see a proper sword-fighting game, and it sounds like there are a few of those in the works, including Red Steel 2 and an over-the-horizon Legend of Zelda. I would be pleased to see a new WarioWare or Cooking Mama game with MotionPlus support, because the controls on these titles' events occasionally feel approximate at best.
I think motion control is still in its infancy. Just like early sound films, when everything was suddenly a musical, videogames still have to figure out how to use this technology most effectively -- the plethora of lame Wii "party games" can be blamed on the appeal AND the limitations of the current technology. I think MotionPlus will help reduce the "waggle" effect, because programmers should no longer have to rely on "Hey, it's moving! I'm not sure what it's doing, but it's moving!" as an all-purpose cue. Confusion between upward and downward movements should become a thing of the past, more intricate hand motions should be reliably picked up, and more information about what the player is actually doing can only help.
I will be interested to see what happens with Microsoft's Project Natal and Sony's wand controller -- what they have shown so far is still pretty simplistic in its approach. But there's plenty of room for experimentation and competition as technology and related game design matures, and I look forward to further exploration of this new frontier in gaming. Nintendo took the big chance on motion control, and it's been a successful experiment -- but we still have a long way to go before it fulfills its promise. Wii MotionPlus is a big step in the right direction.