First of all, there seems to be some confusion how the name is spelt. Some sites have it as "rakuhiki" while others have "rakubiki". Well, according to the furigana on the cover, it's "rakubiki", so that's what I'm calling it from now on.
What you should know first and foremost, if you don't have any prior knowledge of Japanese, this isn't for you. The entire interface is in Japanese, the introduction that comes up when you start it for the first time is in Japanese and the accompanying booklet is in Japanese. It quickly becomes clear that this isn't a E-J dictionary for the beginner Japanophile; this is purely designed for the Japanese. There is no kanji-lookup, nor is there any way to find out pronunciations of unfamiliar kanji you see on the screen. These are the two big failings of Rakubiki Jiten that turns it from a must-have into a pretty-neat-if-you're-interested.
The interface is pretty good, you can use the touch-screen to write words either in English or Japanese (you need to specify whether you use hiragana or katakana) and the dictionary jumps to the words that closest match the one you've written. Very nice first impression, but the handwriting recognition can be pretty dodgy at times (I've often had it see "u" as "v" when I haven't written the little foot/serif on it). Luckily you can also enter words via a small keyboard if your handwriting is horrible. There are various buttons around the writing area; on the left side these are, when writing Japanese, choice between hiragana or katakana recognition, and when writing English, choice between letters or numerals; on the right side you will find a button for alternatives when it doesn't properly recognize the input, eraser for when you want to delete a letter you've written, and last on the right is for switching size on the last character you've written. This can also be used in the Japanese input for making the small versions of characters as in じゅう (十), いっせん (一千) and じゃない (of course you can write them too, for those occasions you can use just a quarter of the writing area, as it's neatly divided, and it will usually recognize them as small characters).
There are four different dictionaries to use (the buttons for them encapsuled in red in the screenshot on the right): English-Japanese, where you write words in English; Japanese-English, where you write in Japanese; a Japanese word dictionary where you can look up definitions of words (in Japanese); and the last one is just everything compiled in one lump. I can also mention here that some of the kanji on the bottom screen can be very hard to read: the third button here looks like it's "工" inside a square followed by a "語". It was only through searching WWWJDIC that I came to the conclusion that it's supposed to be "国語".
When you've written in your word you select it from the dictionary list above the writing area and it appears on the top screen. It's only via the Japanese-English option that you can get hiragana pronunciation of words, so you will need to know how the word is pronounced before you can get the kanji for it. A pretty backwards system for the beginner.
When you have the word looked up, there's a small blue button called "jump" at the top of the touch-screen; with this you can select the next word in the translation, but as it only jumps to English words, this is also near useless.
For situations when the text doesn't fit in the top window you can use the white scroll-bar on the right of the touch-screen to scroll up and down. It's a very natural way of dealing with it and I found myself quickly picking it up. The arrows above the scroll-bar provide the same function, hold them in for fast automatic scroll. In the same way you can use the arrows on the sides of the "book" on the lower screen to quickly turn the page; you can also simply drag the stylus from one page to the other, the similar to the way you can wet your finger and drag a page to turn it.
Next to the question mark (which brings up the help, again in Japanese) there are two buttons marked "大" (large) and "小" (small), these set the size of the text in the top screen. You can also choose between two sizes for the text in the "book" on the lower screen, but for that you will need to press the "tools"-tab, suitably marked with a wrench.
Pressing the tools-tab also brings up a whole range of other capabilities that Rakubiki Jiten has. I don't know what all of them are for (as they use kanji I'm not familiar), but some of them are: Pictochat integration, allowing you to send dicitonary entries over the chat, an timed shut-off function, a world-clock that presumes you're in Japan, a calendar, an alarm clock/timer, a calculator, a small picture-book animation thing that you can use to make simple hand-drawn animations (in the same way you can write small figures in the corners of pages in a book and flip through them quickly to have it move), setting for the dictionary word list, notes from the makers of the books that Rakubiki Jiten is based on (presumably), and a set of English phrases (such as "Is there a post office near here"), presumably for the Japanese to use when they want to communicate with English-speakers without actually knowing any English.
In conclusion, it's a nice piece of software, but there is no way that it can replace a real electronic dictionary. If you already own an NDS, and if you have a real interest in Japanese, then you can consider buying it, but don't get it just for the Japan-trip expecting it to enable you to communicate with the natives. You could probably use it in the way that you write in the word in English and show the Japanese equivalent to the person you're trying to communicate with (in which case it would be faster than a traditional dictionary), but I don't think that solution would appeal to the language student. Don't get this instead of a good Japanese grammar/text book.
A weak 3/5. To be honest I'd rate it 2.5, but my interest in Japanese brings it up a bit.