I currently own a Nintendo DS Lite and was never really interested in trading it in when the DSi came out. Nintendo then announced that the DSi will be giving some steroids to grow into the DSi XL (LL for Japan) with bigger screens and... thats it.
Conceptually you'd think it wouldn't matter cause it'll still be sporting the same resolution and the games will look stretched out. But after seeing and handling a DSi XL for the first time, the bigger screens do make an impact. Navigating the touch screen is a breeze with all the free space and the games do look impressive on the bigger screens compared to my DS Lite.
Now I have heard complaints that games that use sprites don't look all that good since they're stretched out and you can point out each pixel but I have also heard that games that use 3D graphics look superb on the XL. The whole stretched sprites thing doesn't really bother me enough to make me not like it but I completely agree with the statement of 3D games looking better on the XL.
I won't go out and trade in my DS Lite this very moment but I gotta say that if you don't have a DS/DSi and you don't mind the girth of the XL but want to play on the bigger screens then consider getting the XL. I can honestly say that the XL is an easy recommend for me and with the 3DS already out, I'm sure the prices for these past generation Nintendo handhelds will go down in a heartbeat.