A series of rubber bumpers lets the 3DS sit snugly in its new encasing, and there are openings for access to only some of the 3DS' ports, like the headphone jack, charging port, and volume slider. With the Circle Pad Pro attached, there's no access to the SD card slot, game card slot, wireless button, or stylus. The device talks to the 3DS via an infrared port (IR) that lines up when attached. Games that support the accessory will ask for a brief setup when you begin a game.
The Circle Pad Pro turns the 3DS into a bulky mess, adding a good inch and a half in width and even more in height. Anything you've purchased to keep your 3DS protected, like a case or a sleeve, is instantly negated by the add-on, but of course the two can be easily separated.
As ugly and cumbersome as it is, I actually really like how the Circle Pad Pro feels and plays. First off, it improves the 3DS' ergonomics by leaps and bounds. It makes the 3DS easier to hold even if it throws things a bit off-center. But by far the best improvement has got to be the addition of that right thumb stick. Playing a game like Resident Evil: Revelations is a prime example of just how good it feels. So much so that once I experienced playing with it, I refused to go back to the normal setup during the rest of my time with the game.
I've got to imagine that a right circle pad will be included in the 3DS "Lite" or whatever the next evolution of the system might be, but it certainly comes off as a slap in the face to those unlucky souls who decided to adopt early. Sure, not every game makes use of the add-on, but there are a few notable titles coming down the pike that will use it, like Kid Icarus Uprising and Kingdom Hearts 3D.
If reduced mobility and bulkiness aren't concerns, dropping $20 for the Circle Pad Pro seems like a no-brainer. It might not be the most glamorous of accessories, but what it lacks in the cosmetic department, the Pad Pro makes up in the performance department.