The 3DS is so worth it. I wanted a DS, and when I finally got one, I was so disappointed. I just can't find any games on it to keep my interest.
With the 3DS, there are so many games that look awesome, like this, the new Paper Mario, Star Fox 64 3D, MGS 3, Ocarina of Time 3D, Kingdom Hearts, Kid Icarus, Mario Kart, not to mention the other games not even announced yet, to come later in its life.
I'm sure it's not gonna be anywhere near $300, maybe not even over $200, because I doubt they'd price it more than the current price of the Wii. Even so, it won't go over $250. Also, (I think) it's been confirmed that it is fully backwards compatible with DS titles, so it'll be even more worth it. You can just sell your old DS (or DSi or DSi XL lol) and buy more games!
And my guess is that it'll be released in mid-late fall for Japan (around the end of October, early to mid November) and everywhere else in Spring 2011 (March, maybe April).