The DS Lite is a huge improvement over Nintendo's original DS. The DS Lite has a much brighter and colorful backscreen that will allow you to experience all of your DS and GBA games the way they were intended to be played. The original DS had the problem of being large and awkward to hold. However, the Lite does not have this problem; not only is it smaller, but it actually feels comfortable while you're playing, even if for a long time.
Unlike the original DS, however, GBA cartridges stick out of hte bottom of the DS when inserted. This ruins the asthetics of the system and slightly interferes with portability. Touching the system at all will leave oil all over the casing as well, leaving the case greasy. Although the battery life is good, usually allowing me about 10-15 hours with a DS game, the low battery will only activate when about 30 minutes of gameplay are remaining, meaning that if you're away from your charger, you're out of luck.
Overall, though, the DS Lite is a huge improvement over the DS, and I would recommend it to anyone, especially DS owners who are on the edge about making the jump. The better lighting, lighting, and feel make it worth it, especially since a used one can be found for $100 or less now.