This is not out in the states yet but it is in the UK, but I was lucky to get one as a gift for Christmas. :D It's a version of Opera specially made for the DS.
I'm very impressed with how they made use of the dual screens. There are different modes your screen can be in - you can have the dual screens act as one screen, so as you scroll down the top partof the page continues onto the top screen. Or, you can have one screen in full page mode - you cannot see any text unless you have super eagle eye vision, but you can see the general outline of the page. There is a rectangle that you can drag around on this area. Whatever is within the rectangle will be displayed on the other screen in zoom mode.
I'm also impressed with the handwriting recognition. I've had Pocket PC's and Palm, and I have to say this is actually a smarter design. You have two writing areas that can be used for lower case, upper case, or symbols. The character type is toggled by 3 buttons above the writing areas. (this differs from PDAs in that PDAs usually have a seperate writing area per character type.) When you write your character, the software processes it and then displays the character in Arial as a verification. But the purpose of the two writing areas is that you don't have to wait for the processing time - you can continue writing the next character, it's put in a buffer, and by then your other writing area has been processed and cleared. So yeah, you have to switch back and forth, but there is no pausing, even if it's a second, like on a PDA.
The browser comes with an memory cartridge that fits in your GBA slot for, I'm assuming, bookmarks and cache. There doesn't seem to be a way to access the cartridge directly to see what's on it (unless you have a dev tool or something.) If you play another DS game, you do not have to remove the GBA cartridge - it will run just fine.
Limitations - there are limits to what kinds of things you can view on web pages. Most Java or client-side code will not run. More than likely, Flash will not work but I haven't tested it. Speed is another limitation. It's as fast as a PDA web browser, but don't expect anything more. Like PDA browsers, you can set it to enable or disable pictures to increase speed.
All in all, it's fun to have if you are into gadgety things.
Freehand data entry mode
Keyboard entry mode:
Single Screen Mode - both screens are combined to make one long screen
Overview Mode - the full page is displayed on the bottom with a blue box. Area in the blue box is magnified in the top screen. These two screens can be reversed so the magnified one is on the bottom. In which case, you can drag the page around similar to dragging a page in MS Word.