So I picked up a Circle Pad for my 3DS along with my copy of Resident Evil Revelations a few days ago, and having spent some time using it while playing the aforementioned game and with my 3DS, I thought I'd share my initial thoughts on it as well as how it compares to the dual sticks on the Vita.
The first thing I noticed when setting up the Circle Pad was that opening the battery compartment required a screw driver. I didn't have one handy and found this to be a bit of an annoyance since I'd have the same problem if I was using it out on the road and needed to change the battery. I realized later however that the very thick groove was most likely designed to be opened with a coin instead of a screw driver. I didn't read the instructions, so take that as you will, but I doubt most people will either.
The Circle Pad connects to the 3DS simply by pushing the two devices together. It uses the inferred sensor to tell that the device is connected and to send data to the 3DS unit. No cable or metal contact is required and the add on is automatically detected when attached or removed while playing.
When playing Resident Evil the circle pad feels pretty comfortable in my hands and it definitely makes using the trigger buttons easier, at least for me. The circle pads also fit nicely in my thumbs and it feels somewhat natural if you're used to a dual analog controller config. They are still slide pads though, so the feel is a little off. As far as accuracy, there doesn't seem to be much difference between using the two circle pads and the two sticks on the Vita. Both work smoothly when moving your thumbs slowly, but are a bit harder to be precise when trying to make quick aim adjustments. Uncharted: Golden Abyss on Vita compensates for this by letting you aim via analog stick and motion control at the same time, which is really really awesome. I found myself wishing they'd done something similar with Resident Evil Revelations. That's more a critique of the game than the add on though.
The only real problem I have with the Circle Pad add-on, (aside from it not being a part of the 3DS in the first place) is that the placement of the second slide pad feels awkward. It's too far off to the right, and for some reason the way it's positioned makes my thumb stretch in an awkward way that hurts after playing for only a short time. I found myself having to constantly take breaks to rest my thumb which was frustrating. I even considered switching to the regular controls and removing the add-on at one point after about an hour of play because it was really bothering me. I've since found I can reduce the effect by holding the 3DS a bit differently, but it's still awkward. The second slide pad feels like it should be where the face buttons are because of where you have to rest your index finger for the top right trigger buttons.
So in the end it only cost me 1500 yen (about $18) for the thing, and it does make aiming and camera control easier in a game like Resident Evil Revelations or Monster Hunter 3G, but I'm not totally convinced that the pros outweigh the cons (increased size, awkward button placement, reduced portability) in this case. A redesigned 3DS with two circle pads and a better form factor is really a must at this point, and it's just unfortunate that we have to go through all this to get to that point when it should have been included from day one.
Should you get one? I'll leave that question up to you. I don't regret picking it up really, it's made my experience with Resident Evil Revelations more pleasant than I imagine it would have been without it, but I still dislike the thing on principal and for the reasons mentioned above.