The Circle Pad Pro could have easily been awful, given the circumstances of its conception, and from first impressions it looks that way; it's a rather ugly, misshapen thing that only looks to add width and bulk to your unassuming 3DS.
Luckily, it performs it's intended function well. Having used it in tandem with RE: Revelations, I can happily say that it makes the game much faster and easier to control. Whilst the regular controls sans the Circle Pad are relatively well-adapted considering, the accessory makes things much easier and far more fun. There's no noticeable lag between the device and the console, and the design is rather more ergonomic that first impressions would have you think (although the size and button/pad placement shift do take a bit of getting used to initially). Build quality seems good, something that should be expected from Nintendo by this point, although it feels oddly hollow even with the battery in.
Should you buy it? If you're planning on getting Revelations, absolutely; it's worth the extra little bit of cash to get fully-formed controls. If you've no interest in RE, then there's little point in having it right now, so wait until a title comes out that is compatible (Kid Icarus, KH3D, Dynasty Warriors Vs.).