OK, here are some impressions on the 3DS wheel. I've been using it since launch day and it's my preferred way to play Mario Kart 7, and pretty much my preferred grip for many other games. It makes the 3DS more comfortable to hold for long periods of time.
The most important part of the design for me are the bits that stick out at the bottom, the "legs" as I will call them. These legs actually give the last three fingers (those other than your index finger and thumb) on each hand something to hold on to, and prevent the corners of the device sticking into your palms. It totally removes the pain issue as well as the possibility of getting pins and needles - at least in my use.
The shoulder triggers are a nice bonus, and have almost as much movement as the standard triggers - definitely no more movement than the standard as they simply press the standard triggers down mechanically. They are hinged towards the middle of the unit which means they feel more responsive towards where your fingers are (see photos). They have a different feel and size, which can take a small amount of getting used to.
The circumference of the unit is good. All in all the wheel a couple of cm less wide compared to the Wii wheel (see photos). The thickness of the grip itself is slightly fatter than the Wii wheel. The curve of the grip is rounded on the back side of the wheel, and flat on the front.
You put the 3DS into the wheel shoulder buttons first, and there is some soft rubber to make the unit fit perfectly just above the legs. All in all, it's excellent. Everything you would expect from an officially licenced peripheral, and up to Hori's high standards.
One last thought: I think this makes the game feel more like a home console game rather than a handheld, which is great.