Mine finally arrived today.
I played a single playthrough in "3DS Mode" with the N64 controls. As far as I can tell, 3DS mode is just a bit easier than a normal N64 playthrough. They also offered "N64 Mode" which claimed to offer the original StarFox 64 challenge.
Sound: Upgraded. I prefer the old N64 sound, but it wasn't bad. Just... better sampled instruments. I didn't really notice too much difference with the sound effects. The voice acting was definitely different. They seemed to work pretty hard to keep the voices and acting true to the original, but there were a few differences. Not bad, though. A little better in some places, a little worse in others. I didn't find myself getting as annoyed by my teammates. ROB 64 was quite different, though.
Graphics: Upgraded. Models were higher-res, most of the textures were better, character animations were slightly better. I was impressed by the texture of the water(?) in Zoness. One complaint is the draw distance... I noticed quite a bit of pop-in that I don't remember from the N64. Aquas seemed to stutter a little more than the N64 version, and I saw stutter in a few other spots. I did not find that the system slowed down during the use of a bomb. Pleased that the 3D effect almost never bothered me, unlike in Pilotwings Resort. The 3D was generally good, although none of the attacks that fly at the screen really impressed me. During the hard mode battle with Andross, when his brain was chasing me around the stage, I felt a presence right in front of me, between the screen and my face. It was rather interesting, although I think it was unintentional. Either that or done well enough so as to seem unintentional.
Controls: The Circle Pad is quite responsive. Generally, the controls felt good. I liked that they offered an option for inverting the Y-axis (which I decided against) and that they offered an N64 control scheme to mimic the original layout of the N64 controls (Y for bombs, X for boost, A for brakes, and B for lasers). They also offered a more intuitive option for new players: to swap the brakes and lasers, making brakes B and lasers A.. I chose to stick with the N64 control scheme so that I could still use lock-on bombs. Gyro controls were pretty good, too. You can recalibrate them on the fly by tapping the brake button. I only occasionally relied on the motion controls... normally just using them in concert with the Circle Pad. They also made the D-Pad able to perform your somersaults and U-turns (by pressing up and down on the D-Pad, respectively), and to change the perspective of your craft by pressing the D-Pad right. Incoming messages are received by tapping the screen. Easy and intuitive.
Gameplay: Pretty much the same StarFox you know and love. Nothing immediately makes me say "run out and buy it." One minor annoyance was the occasional reminder to take a break from playing every so often... but it was only a minor annoyance. I do like that they added an auto-save feature. If for some reason you want to play something else mid-game, it will save your progress after each world you beat, and you can continue or start over from that point the next time you play the game. It also offers a "guest game", presumably to allow a friend to play without writing over your game in progress.
Multiplayer: I have yet to find someone else here at school that owns a 3DS. Frankly, I'm not holding my breath.
All in all, it's a fairly faithful port with some minor upgrades. If you want to play Star Fox 64 on the go, this isn't a bad way to do it.