I've heard the DS is a bit slow. I guess its OK if you're not in a hurry. I use a Sharp Papyrus, it works well, no problems.
I want to butt in mainly to make a point towards freeware.
It is perfectly possible to get a palmtop or a dictionary sized laptop (I'm not sure what they're called but I saw some in Bic Camera last time I went, each running windows XP or Vista) They're a bit more expensive, but have lots of advantages:
1)It would be childsplay to install a free dictionary onto such a device (there are even websites with instructions on how to do this). And since most of those devices have pointing devices you also get to write the kanji in, just like the good Denshi-Jisho's.
2)You have the ability to use an English operating system, so no previous experience in Japanese would be necessary.
3)The best thing is that you basically end up with Jim Breen's dictionary on your palmtop(/other mini computer) and all of the scientific/literature/name dictionaries you'd ever need.
4)All the features of a tiny computer (who needs those brain-training programs anyway
)
Hope I don't sounds too much like a electronics sales rep. I'm biased somewhat by the need of a good scientific dictionary