I got one yesterday so here are my thoughts so far.
It's a nice upgrade, but it's not completely practical if you already have a regular 3DS (XL)/2DS. I can see it being amazing for those who desperately need a built-in Circle Pad Pro for games like Monster Hunter which require ******** of camera movement and don't want to use the D-pad to do so. I only got a N3DS because the Circle Pad on my old one was starting to go bad, probably because fifty hours of Smash Bros. on it messed it up badly (and I'm usually pretty gentle with analog sticks.) That, and the regular 3DS is actually getting kinda small for my hands.
I find it nice because the faster CPU and doubled RAM/VRAM means games load faster, the internet browser is actually almost as functional as the one on the Wii U (though, it still chugs on Gaia, but at least most of the graphics load,) and the same goes for Miiverse. buuuuut I can't use Miiverse because my main account is permabanned because of the super trigger-happy admins on there who will shoot you for dumbest reasons.
The only thing I absolutely HATE about it is the way the microSD card is accessed. I'm personally fine with the switch to regular SD's to the micro ones because they cost about the same, but I hate how you need to unscrew the back cover to gain access to it. I bought a 16GB microSD card for it, and I switched out the preloaded 4GB one and put in the 16GB one before I booted up the system for the first time, and I accidentally twisted the screws too tight when I put the cover back on. I actually expected the system transfer process to be incredibly easy, seeing as the tutorial video Nintendo made for using the "Transfer with a Computer" method made it seem as if anyone with basic computer knowledge could do it in like, five minutes, man was I wrong. When I needed to get the microSD card out to get the contents of my old 3DS'es SD card transferred in there, I stripped the crap out of the screws trying to get the back cover off, because I was stupid and decided to screw them back on tightly, so that was my fault on that part, had to use the world's smallest flathead to get them unscrewed.
And here's the part the scared me quite a bit. When I got my stuff transferred, it didn't pick up any of my downloaded games or anything. It wouldn't pick up my save file for Fantasy Life, because that game saved data to the SD card instead of using SRAM like most games. I thought I had just lost fifty hours on that game, and another fifty on Smash Bros. It may not seem like much, but Fantasy Life requires a lot of grinding, and unlocking a lot of stuff in Smash Bros. is a pain in the a**, especially on the 3DS version which feels kind of awkward to play on a handheld compared to a console. Luckily, I copied the folder from my old SD card instead of moving it. So I deleted the contents of the 'Nintendo 3DS' folder on my microSD card, and copied the contents of the same folder on my regular SD to there, and sure it enough, my New 3DS came to life with my Primal Groudon/Kyogre theme that I had set on my other 3DS, and everything like my Eshop games, and my Fantasy Life and Smash Bros. save data was all there on this new system. Needless to say, I was relived, and honestly, I should've done a wireless transfer to save myself a big headache back there.
I apologize for the long rant, but it frustrated me a lot more than I anticipated.
Also, the C-stick is a lot like that eraser head mouse pointer on the IBM ThinkPad laptops, so it may feel like the thing is stuck. I don't have a whole lot of games beside Smash Bros. that makes use of it, but I can tell it's gonna take a while to get used to. The screens are still at 240p, so everything is still gonna look like an Atari 2600 game to anyone who can't tolerate any resolution lower than 1080p. I can't comment on the improved 3D, because I lack a fully functional right eye to actually see any sort of 3D whatsoever.