So I've been messing around, and my takeaway so far is this:
Sega CD emulation is freaking incredible on Picodrive. Like the gods are smiling down, blessing me with pixel perfect Sonic CD, Lunar, and Popful Mail. Unreal. Why is the genesis family so smooth and freely emulated? Even by Sega themselves.
PCE CD emulation is pretty bad. Unfortunately, the sweet old-school anime cinemas run fairly slow with glitchy sound. The gameplay parts run ok, but I really just want sweet cinemas and redbook audio in truth. This is my biggest disappointment, as I adore PCE CD.
SNES emulation is mostly good but with some spotty drops in framerate, like Super Metroid at the ship it drops down to 45 fps or so. But mostly the bomb.
NEO-GEO emulation is incredible, as is CPS1 and CPS2. Really smooth, excellent framerates. Loving it.
That's about it! I'm LOVING my 3DS and with this emulation I'm DOUBLE LOVIN IT