Bring a phone or buy in Japan?

A soon to depart JET left a comment the other day asking my opinion on what to do about a phone in J-Land. Hereshigoes writes:

Hmm. Very interesting. 

Question for you though: with all this knowledge and your experience in Japan, do you recommend just getting a Japanese phone there when expecting to be there for a year OR [drumroll] unlocking your U.S. phone for like Docomo or something? 
I've heard SO many yes and no's but I am not sure who or what to believe so I don't know what to do. Its annoying. 

Advice?

I replied to her comment, but for a more complete answer I've written up a guest post over at the JETAADC blog. Have a look if you're interested!