I had one that I used for Resident Evil Revelations and MGS for my original 3DS but I traded it in when I upgraded to a 3DS XL. I am awaiting the release of the Circle Pad Pro XL in NA (as ugly as it is). I can't play either Resident Evil Revelations or MGS without it...

I bought one cause I have RE Revelations and Metal Gear 3.
RE actually feels WORSE with the circle pad pro, but it makes aiming easing in metal gear. Not that it feels like "needed" or anything like that.