It really depends on what level you're at and where you want to go with your Japanese study. I just bought the Wordtank G70 a few months ago and I really like it. It has lots of dictionaries and is pretty easy to navigate. The good thing about the Cannon models is that you can switch the menus and things into English, which is handy if you're at a beginner level.
Before the g70 I had a g50... the g50 is no longer available... it's been replaced by the g55. I really liked the g50, and when I bought it a few years ago it really suited my needs well (I'd been studying for about 2 years at that point). I did start to outgrow it a bit before I bought the g70 though, as I felt I reached a point in my studying where I needed something with more entries and dictionaries.
My recommendation is to evaluate your study/use goals... I've found it worth the investment since it's something that I get several years use from (I'd actually still be using the g50 if wouldn't have accidentally stepped on it when I got up in the middle of the night one night
) Once you get that taken care of, start looking around. When I was doing my shopping, I found the Japan Shop's guides and recommendations pretty helpful.
Recommendations:
http://www.thejapanshop.com/recommendationsjisho.htm
Comparison chart:
http://www.thejapanshop.com/compare.htm
The comparison chart is a little out of date, but it should give you a good idea of what's out there. I've seen cheaper (3,000 yen - 4,000 yen) dictionaries at bookstores in Japan, but not sure where you could get them online. They're really basic and would probably get you by if you're not looking for something to use with studying.
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