Here are my early impression after playing some games for only ~10 minutes each.
I played Tales of the Abyss, Dragon Quest V (native mode), Resident Evil: Revelations, Theatrhythm and Ocarina of Time. I've played all of these games before on my regular 3DS. Some of them was a long time ago, but others are more recent. I'll try and make some comparisons.
I don't use 3D at all, so I have no impressions about that. Also, I'll refer to the regular 3DS as "3DS" and the 3DS XL as "XL".
First, I played some Tales of the Abyss and I felt that the screens seem a bit darker than on the 3DS, so I had to raise the brightness from my usual 3 to 4. The game looks pretty good, and I can't say that I have any major complaints about how it looks on the XL.
After a few minutes I decided to try the game on my 3DS and I could tell right away that it clearly looked better on the smaller screen. If you've never seen the game on a 3DS, or at least haven't played it on a 3DS in a long time (like me), then you'll think that the game looks just fine on the XL. The intro is clearly pixelated, but other than that I had no problems.
After that I decided to try Dragon Quest V in native mode. I guess it looks just like it would on a DS Lite or DSi. The new dpad is waaaaaay better than on the 3DS. It's not the best dpad I've ever used, but it's a big improvement over the one on the 3DS. In fact, all buttons are improved on the XL.
Resident Evil: Revelations looks ok I guess. It's nice to play a game like this on a bigger screen, but at the same time it's really going to show the flaws. It's just one of those games that won't look great on a bigger screen.
Next up was Theatrhythm, and this game looks really nice. The colors definitely pop more on this screen. The game plays pretty much the same as on a 3DS. I had some worries about being forced to make bigger movements for the slide triggers, but they registered perfectly with the same amount of movement as I did on the 3DS. The hold triggers feel a bit weird at first, but I quickly got used to them.
Ocarina of Time was probably the game I felt benefited the most from the bigger screens; at least out of the 3D games. Some of the 2D assets are very pixelated, like the map, but overall it looks good. A game like this was really meant for a bigger screen than what the 3DS has to offer. It's a shame about the low resolution we got, but it still looks pretty crisp and I immediately felt that I wanted to start up a new file and go through the game once again.
So, overall it's a mixed bag. The games will obviously look crisper and less pixelated on the 3DS than on the XL, but they still look pretty good and some games feel really nice on the bigger screen. Some things, like the home screen, looks pretty damn bad with the low ppi.
I love the new buttons and dpad, and the new position for the stylus is a million times better.
I haven't yet decided if I'm going to keep this and sell my 3DS or the other way around. I'm a bit worried that the XL might be a tad too big for my small hands, and I'm going to really test it out once I charge it. I'm going to play Mario Kart 7 for an hour or two and see how my hands feel after that.