Well, here's the thing. On paper, the Lite is obviously superior than the original DS. Brighter screens, slightly better battery life, more compact design...
But...that's simply not the case. The DS Lite is extremely flimsy, and it'll malfunction on you some way or another, no matter how well you treat it. Whether it's the L/R buttons dying out, or your hinge cracking for no reason whatsoever (you don't have to look hard to find horror stories of hinge cracks. Just google image "DS Lite hinge" if you don't believe me.), the thing is plagued with problems. Also a minor annoyance, but not a true problem, if you have a GBA game in the Lite, it'll be sticking out of the bottom of the system, instead of fitting correctly like it does isn the original DS. The d-pad and buttons also feel much worse than the original's.
On the other hand, the original DS is extremely sturdy, it's just as tough as the old GameBoys. The only real problem with it is that the screens seem pretty dark in comparison to the other models'. But you won't even notice unless you play a newer model to compare it to.
The verdict is...no, I don't think it's worth upgrading to a flimsier device if your current one is still going strong (which I'm sure it is).
The DSi/XL on the other hand, is arguably a true improvement. Bigger screens (even moreso if you go for the XL), an even better battery life if you get the XL, downloadable games and programs, and much more make it a worthy upgrade. However, the DSi/XL has no GBA slot, so if that's really a problem for you, you should stick with the original DS.
Lastly, there's the 3DS....which isn't out yet, and probably won't be for a few months. It's not a revision, like the Lite/DSi/XL, but rather a next-generation handheld. It is, however, 100% backwards compatible with DS/DSi games, along with having its own library of games. Still no GBA slot, though.