Being the proud owner of no fewer than three DSi's (there's a very good reason for that, mind), I'm perhaps perfectly positioned to offer an honest opinion on whether it's really worth upgrading to if you own a DS Lite.
The answer might surprise you a little bit: it's a 'yes', but not without considerable caveats. For example, I'd only really recommend it to those who never use their GBA cart slot, or who have another way to play GameBoy Advance games. If you really can't do without your GBA collection and money's no object, then get a GB Micro and a DSi - best of both worlds. Luckily, I already owned a Micro, so it was a no-brainer to get rid of my Lite, really.
Secondly, even if you've never used your Lite to play a GBA game, the trade-in value of your Lite compared to the RRP for a DSi doesn't make it a massively tempting upgrade. There's absolutely no good reason to owning both consoles, so if you are planning to make the leap, then selling your Lite or trading it plus cash for a DSi is the way to go.
Ultimately, you've got to want the additional features the DSi provides, and I'm not sure there's an awful lot that's worth buying a new console for on its own. The music player isn't comparable to a standard MP3 player, and you can get better cameras on your average mobile phone. Sure, it's nice to manipulate images in real-time, but that's a fun novelty and nothing more - at this stage, it's essentially the gaming potential of the two cameras you'd be paying for.
Well, that and the DSiWare - most of which could easily be produced on the Lite, but that obviously doesn't have the capability to play downloaded titles direct from its memory. Still, even with the vast majority of DSiWare software making little compelling use of the console's unique feature set, there are a host of titles well worth spending your points on. Just about every Art Style title of the five (soon to be six, hopefully, with Art Style: BoxLife surely just around the corner) with the possibly exception of dull tile-matcher Aquite is worth the 500-point outlay, with special mention for the incredibly addictive KuBos and PiCOPiCT, and the mind-bending Code. A Little Bit of...Dr. Kawashima's Brain Training is also (perhaps surprisingly) worth 800 points - Brain Training was always going to be perfect for DSiWare, as it is structured for daily play, making it a valuable addition to your DSi's internal library. Easier than swapping carts over, isn't it? Besides that little lot, there's nothing else we'd give a wholehearted recommendation to, but with a series of 200-point snack-sized minigames, there's bound to be a gaming canap you find that's to your tastes - and it's still early days, after all.
So, ultimately, it's down to what you want from a console. If you're excited by the possibilities of downloadable games tailored for quick-fix gaming on the go, happy to pay for the potential of future camera-based titles, and don't mind losing the ability to play GBA games, then by all means take the plunge. Otherwise, save your money until something truly compelling - something that can only be done on DSi - comes along.