I don't know if the DSi is worth the added price because I don't have one and I still have the ORIGINAL DS. But frankly, I'm going to get the DSi XL when it comes out no matter the cost. And my suggestion to you and anyone who doesn't have a DSi yet is to do the same. The DSi XL is suppose to launch in America sometime in 'Early 2010' for America and Europe.
As for a DS being worth the cost in general, any DS is worth the cost. You can't imagine the amount of playtime I have gotten out of my original DS in the last 5 years. I bought it for 250 dollars a launch and now you can get a DS Lite for around $125. And in that time, I have probably logged more hours playing my DS than all other gaming platforms combined. It should be a no brainer for everyone to buy a DS. That's the price of less than 3 console games for practically the best gaming device of this generation. Though I still recommend you wait for the DSi XL (which will cost more).