I've looked at kanjigym and PAdict on the websites and as far as I'm aware, they don't offer anything special. I know that Leo is using Supermemo and I believe that there are supermemo versions on both palm and pocket pc.
I got an HP iPAQ hx4700 a couple of years ago and it's been fantastic. If you buy one with an English OS, then you won't be able to view Japanese characters or input them. From the above post it seems as though it's the same for palms. If I were going to buy it again then I'd get a Japanese OS one but since I didn't do that, I installed some CAB files that were created by a guy called Bagoj. These made alterations to the wince.nls file and some registry entries that resulted in Japanese input and viewing capability. For input, a number of IMEs can be used on my iPAQ. One of them involves entering characters by radical and there are a few others but by far the best is the multibox. This allows you to write the character in a box and the IME recognises it. It works very well and even if you use a more cursive style, it will usually recognise the character. My only complaint is that I can't find a way to write つ and っ so that the IME always differentiates.
As you'd expect the IME on the pocket pc looks very similar to the IME on a normal windows machine. You get a small window which is very similar to the language bar and allows you to select between hiragana, katakana and romaji input. This means that when I use my bluetooth keyboard, I can type away in Japanese in the same way that I do on a windows machine.
For software, I use JWPce. With it's built in dictionaries and look up methods, it works very well. It doesn't have stroke order diagrams like PAdict but apart from that it has just about everything. Especially when you have the multibox IME to enter characters. You can use JWPce to create flashcard files for both JFC and kingkanji.
I used to use kingkanji a lot but it has an extremely simple method of review. You see cards randomly and if you get them wrong you see them again. Simple as that. The advantage of king kanji is that it gives you feed back on your stroke order, which is great. However, it only has the stroke order of about 1000 kanji programmed in so not that brilliant.
For a more sophisticated reviewing algorithm, I moved to JFC. This shows cards you get wrong a lot with higher probability. It's not Leitner but it's better than kingkanji. Then I moved to the PC to use this site with the leitner system. Now I use supermemo. I haven't tried supermemo for pocket pc but I would be interested to see whether I can get it to display Japanese characters. Supermemo is an unwieldy beast on windows so goodness knows what it would be like on palms or pocket pcs. I might try it out though.
Nowadays, the pocket pc/windows ce OS's have evolved to Windows Mobile 5.0. The big advantage here is that most of the PDA machines with this OS can also function as a phone. With skype releasing more and more PDA versions of it's software, getting a PDA/cell phone with WiFi and skype could save you a lot of money. It might even work with these new 3G networks that are in Europe (and possibly elsewhere). I'm not certain about this though.
I just moved back to the UK and I got an HTC TyTN. This has windows mobile 5.0 so it functions just like my iPAQ. It's much smaller though and really does fit in your pocket unlike some of the other PDA/phones. It also has quite a nice camera on the back. I'm thinking about installing files like I did with the iPAQ in order to give it Japanese input and display functionality. A bit of work has been done to these files and one guy appears to have included support for Chinese and Korean as well as Japanese. I haven't tried it out yet though. Check out the link I posted above. If I do manage to get it working then I'll be able to email my friends in Japan just like I did in Japan with my Japanese mobile phone. I can already do this in English and I can also send pictures attached to the emails. With windows mobile 5.0's new support for push email, things are promising.
*For those that might not know, Japanese cell phone networks issue an email address as well as a phone number with every cell phone they contract. You send emails in much the same way as you send SMS text messages in Europe. None of that 160 character limit rubbish.
My best advice for choosing a device would be to check out lots of reviews before you buy. I like to go to
http://www.pcpro.co.uk as a starting point. You get really good reviews but sometimes they are a tiny bit dated. (They currently recommend the i-mate KJAM which was the model before the TyTN).
Edited: 2006-08-17, 9:04 am