I've upgraded to the Nintendo DSi XL from a plain ol' White Lite unit, and I could not be more happier if it made the coffee. The XL is a beautiful bit of kit, perfect for young and old alike (I'm only just pushing 21 and I love the idea of a bigger console). Regarding portability issues, if you're someone who only uses their DS at home or sports a backpack/handbag everywhere you go, the XL is perfect. If you're someone who loves shoving their DSi in their pocket before trekking out the front door, you may want to think about just getting or sticking with the smaller DSi - the XL was never really meant to be portable.
A few of the many features which are a definite improvement:-
01) Larger screens. Lots of people have been concerned that the larger screens would make games appear stretched, blocky or pixelated. Not so. The larger screens are beautiful, bright (I use my XL on Brightness Setting 3, and whilst it's slightly darker than the factory setting everything still looks bright and beautiful!) and the new LCD viewing angle is perfect for games played at a table, etc.
02) Speakers. Both DSi systems boast a music facility (Nintendo DSi Sound) where m4a format music tracks can be put on an SD card and played through the DSi XL. You can then muck around with them if you like (the facility boasts a surprising number of gadgets for doing so), change the visualisations much like on Windows Media Player, or just sit back and listen. One of the XL designers mentioned on the "Iwata Asks" interview that not only are the XL's speakers larger than the DSi's, but they have added more holes, which adds to the sound quality. And it really does - songs are loud, crisp without losing their bass, and the speakers work well with both instrumental and vocal tracks. The unit can also turn off its backlights after it detects you're idle to save on battery life, which is a treat.
03) Connectivity. Both my Phat and Lite DSes have had trouble finding our modem from my bedroom (which is only a stones' throw away from our wireless modem!) and have done since I first got them. The XL, on the other hand, has no problems at all. The wireless signal strength is definitely stronger and faster than previous consoles. The internet browser, powered by Opera, is a smidgen slow but perfect for doing on-the-go Google searches (even image searches are processed in a breeze), and the browser is also perfectly capable of displaying Amazon, LiveJournal, eBay and Nintendo itself. The only downside is a lack of flash player (which means you can't access sites like YouTube or Google Video) but really, for a utility to just look things up quickly when you don't want to boot up your laptop, it's a dream. Having a larger screen means a larger keyboard, too, so typing is made remarkably easier.
04) Stylus. The larger stylus that comes with the XL is marvellous. It's the comfiest thing I've ever held, stylus or pen alike, and the little tab on the side stops it from rolling off to God knows where. Sometimes I forget I'm even holding it.
05) Battery life. The XL has an improved battery life over the regular DSi (harking back to the Lite), and on Brightness Setting 3 you can get approximately 9-11 hours (I've tested this, it's true). If you're a battery-conservation freak like I am, you can also turn off your wireless whenever you know you won't be using the DSi Shop or Internet Browser. Just go Settings > Wireless Connections > Turn Wireless: Off. The little yellow light on the left hand side of the top screen will turn off.
The top lid is a bit of a finger print magnet, as the Lite was before it, but it's easy enough to go over with a bit of cloth if you're worried about appearances. The matte bottom is great for holding the console and not getting sweaty hands/having it slip out of your fingers. The Start and Select buttons have been enlarged (so you don't need to press on them as hard as you had to with the Lite), and the moved Power button means fewer accidental turn-offs.
The inbuilt software is a bargain - Doctor Kawashima's Brain Training: Arts Edition is practically a full program if you don't have the original Brain Training cartridge that made the DS so popular, and whilst the Dictionary has a few words missing I imagine it would still help if you're stuck on the crossword. Having the option to look up words in German, French, Spanish, Italian and Japanese is also sort-of cool, if you're interested. (It helps my Dad with decoding his German military history, which is good...) I would also recommend signing onto the DSi Shop and downloading Flipnote Studio (it's 100% free). A lovely little program for writing memos or doing the odd doodle when you're bored. Note that if you connect to the DSi Shop before March 31st, you get 1000 free points to spend on games. I highly recommend Art Style: PiCOPiCT and Starship Patrol if you're into Puzzle/Strategy games.
Finally, the camera is nothing to shout about, but it's great for taking photos to be used on your top screen a la a wallpaper. It's also great for photographing my dog - she doesn't realise it's a camera and sits still for once! The photos can then either be uploaded to FaceBook using the XL's inbuilt internet, or sent to the SD card to be transferred to a computer. The resolution isn't great, but the XL was never made to replace your digital camera anyway.
In conclusion: A vast improvement on its predecessors, perfect for people who are vision-impaired or just want larger game screens, beautiful speaker quality, great wireless connectivity, better battery life, great in-built software. A great machine. I hope Nintendo continues along this path - bigger is better!