The Casio models are very popular for Japanese students and foreigners learning Japanese. The top of the line Casio model provided the green goddess dictionary both eng-jap & jap-eng, nhk pronounciatiation dictionary, and lots of other great stuff. But there is a lot of stuff bundled that is useless. You can buy a lower model and subsequently add propietary dictionaries but they are expensive and I read they can be a bit awkward to use vs. using pre-installed dictionaries. I have limited experience with the Casios so others will be able to add more value here.
That said, I think the Casios are made for Japanese so they are not too useful for foreigners with limited Japanese skills. They are also bulky (and expensive if you want to have a variety of dictionaries).
I use some dictionaries on the iPhone; convenient as phone is always with me. Be warned that the iPhone does not recognize kanji character input (huge fail by Apple). You can use Chinese input but this misses a lot of Japanese words. Writing with you finger is also painful; you can't use any fine pen to draw in detail, just a fat finger or special fat pen.
For the iPhone imiwa is a good free dictionary app. The dictionary is decent but the interface and conjugations were well thought out; impressive. There are a few dictionary apps that have a Japanese kanji input engine (that works only for said programs); those are discussed on this site recently.
My main dictionary is the Dell Axim x51v. It is a PDA (!) with a great kanji input system and small enough to carry in my pocket, although it is not svelte. It uses EB Pocket as a dictionary app and I upload dictionaries. I use this as an electronic dictionary and have some Japanese databases on excel (eg heisig keywords). Microsoft just Crushed the kanji input software for Windows Mobile 6.x; the kanji input is superb and an absolute pleasure to use.
Actually also I have EBPocket on my iPhone and desktop, all running the same dictionaries. It is a good solution but for great Kanji input, I rely on the Axim.
The guy who sells the Axims and related software also recommends the Samsung Galaxy Note. The Galaxy provides great features in a modern android platform. Getting Japanese Kanji input to work in Androod is also a challenge (Google fail) but links below and some links at RTK site can help.
If you have a Nintendo DS, there is a Japanese-English dictionary program that sells used for about 2000¥. It has good kanji input. But it is only one dictionary and not spectacular. The kanji input is excellent although the screen is a bit small and low resolution.
Some of the following site info is dated but the site is a good resource. There are advantages to buying software that is not locked into one device.
(I am not affiliated with the following site)
http://japaneselanguagetools.com/GalaxyNote.html
http://japaneselanguagetools.com/bus.html
http://japaneselanguagetools.com/docs/other_dics.html
In the end I purchased an economical dictionary solution and used the extra cash to buy some good textbooks and reference sources (Grammar dictionary series, pronounciatikn guide, JLPT books...). If I outgrow my current dictionary I will update that. Cross that bridge when it comes...
Good luck!