Got my black N3DS XL a couple days ago. System transfer took a while, but everything worked. Few thoughts in the short-term, which echo what's already been said. Oh well:
The glossy finish is definitely more along the lines of the original 3DS. It's a fingerprint magnet, but it does seem less slippery than the smoother finish of the standard XL. The gunmetal color does remind me of my GBA SP2.
Swapping the MicroSD card was a cinch. Does suck that you need the #0 screwdriver, but the two screws only take a few turns, and the back pops right off. Remove the stylus first to make it a little easier.
Beefier stylus feels nicer and more comfortable to use. OS and loading in general is much faster, which makes everything feel a lot snappier.
The stable 3D is amazing. I don't know what I was expecting, but it works exceptionally well. You can confuse it by moving too fast or whatnot, but it's crazy how nice it works. Makes me want to play all my old 3DS games with the slider on!
Volume relocation is very welcome. Used to hit it all the time by accident on my old XL.
New Start/Select buttons are also much better.
Power button placement is funky. I don't use it a ton since I generally just sleep it most of the time, but it did take me a couple seconds to find it. LOL
Hinge is so much quieter now, thank goodness!
Only thing I really dislike is the cartridge slot location. My left pinky finger tends to support the system on the bottom edge, and so it's pretty uncomfortable. Gonna have to adjust the way I hold it a little bit. Much preferred the old location. That, or I wish the slot were in the middle-bottom and the headphone jack on the left.
Overall, I'm happy with the upgrade. Haven't used the c-stick for anything yet besides 3D Galaxy Force II, but I will soon.