So I want to say up-front that I'm not the hugest 3DS player, but have owned a 3DSXL for awhile, and just "upgraded" to the New 3DSXL. While I've only been using the new device for a short time, there are some attribute that really stand out. As such, I figured it would be fair to offer up a few initial opinions on it.
Firstly, they really aren't over-hyping the Super-Stable 3D. This new face-tracking image adjustment improves the 3D experience to a profound degree. On the old 3DS, I left the 3D slider on the 'Off" setting almost exclusively. The oddity of the Nintendo's 3DS implementation and the extreme difficulty of maintaining the proper distance and viewing-angle made it a horrible proposition. My eyes would begin hurting almost immediately. With the new face-tracking 3D, it's a pretty seamless experience. Unless you put the 3DS down completely or move your head way-beyond a comfortable viewing angle, the 3D functions without a hitch. That said, I do still get a bit of eye-strain from using it, but it's easily a day/night difference that has allowed me to use the device with the 3D setting at it's max from moment one.
The improved processing power is a more subtle improvement, but can definitely be felt during game loading and settings menu responsiveness. Pressing the home button while in a game also feel substantially more responsive.
I haven't really played anything yet that makes proper use of it, so I can't speak to the C-stick one way of the other.
Unfortunately, nearly every other aspect of the system is a negative for me. The outside finish of the New 3DSXL is an absolute abomination. Where the old system utilized a flat matte finish, this newer model has an ultra-glossy exterior. In addition to being a finger-print magnet, the system just feels worse to the touch. It also gives the system the appearance of having that ultra-thing plastic wrap that ships on consumer electronics these days that was never removed. It looks both cheaper and tackier. The interior has the same black color, but foregoes the ultra-gloss coating. It does however have a "speckled" appearance to the pain that makes it read as slightly shinier than it is.
Overall, the build quality seems a bit more questionable. The system itself feels a bit less solid in the hand. Also the paint on the interior of my unit feels rough in spots, as though tiny drops of paint dripped onto it and dried. My OCD rages at me to get the sandpaper out every time my finger brushes one of these spots.
I personally find the new locations of the power button, stylus, and game cartridge slot to be awful. The power button especially is tougher to press. I frequently use the 3DS while lying in bed, with the system resting on my chest or stomach. This position renders it impossible to quickly remove the stylus like I could when it was placed on the right side of the device. The new extra-flush styling of the stylus and it's slot make it slightly trickier to remove in general. The wireless button has been completely removed in lieu of a software toggle. And the SD card slot has also been removed. Changing the card now requires the removal of the devices rear cover with a size 0 screwdriver (hardly the most common).
While not a factor of the system itself, I will say that the system transfer process (which is necessary if you're upgrading from a previous system) is completely absurd. The number of steps, the need to frequently swap back and forth between systems to carry them out, the poor wording and redundant explanations, and the agonizing amount of time it takes to transfer wirelessly and far too much for a child to carry out. Add to the fact that new system comes without a charger meaning that it's all to possible to have one system or the other died in the midst of a lengthy wireless transfer, rendering your content unusable. This is the most obvious example yet of how outrageous it is that Nintendo has yet to adopt a proper account system.
In short, right now the main selling point is the improved 3D. Once a larger number of must-play games support the new C-stick and additional top buttons, it will probably become a required purchase for the Nintendo fan, but for now, it's tough to recommend the upgrade.