TGWeaver, thanks for the info on Genki-- maybe I'll save some pennies and head back to beautiful Fukuoka! I'll try to give a summary of my Yamasa experience here, overlong but with headers for navigating ease.
Yamasa is accredited and has a lot of people going through for work or university, so it's pretty intense academically. Probably the largest single nationality is Taiwanese, but no one group dominates-- my class had Europeans, Americans, and Asians. It also seems to get a LOT of interesting people going through-- Nordic metalheads, HK fashionistas, young folk and people in their 50s. And everyone got along great. I hate competition in language learning, and while I saw a little evidence of it, it was never in my face. There's just a great vibe there-- being there reminded me of everything I love about Japan, rather than the bad stuff.
ACADEMICS
They have two tracks, one "academic" (AIJP) and one more conversational (SILAC). Both are all Japanese all the time-- even if you don't like school, you will learn Japanese just by being there. I can't speak to SILAC curriculum, except that it works, and it's what people studying short-term seem to do. AIJP has a three-month semester system, with classes from A (top) to however low they go-- I think we were down to Q or S. I was in the middle

but the class rankings are vague-- we had people who were taking the 1kyuu and 2kyuu both in my class, and another the 3kyuu(!) It seemed standard for people who came in with NO Japanese to get up to the top 3 or 4 classes in a year and a half, more or less. That kind of amazes me.
You'll have one main teacher and then a rotating cast of other teachers, five or six. 20 hours a week, plus four electives (IIRC). The electives are with other teachers, 1 hr/week on a certain topic (kanji, conversation, keigo, etc). The main curriculum in AIJP, at my level, anyway, followed the New Approach textbooks, which have an essay for each unit used to present grammar points & vocab. One of my teachers told me as an aside that she thought the grammar explanations were too detailed, and you'll get a lot of explanations of nuance.
The methodology is very Japanese-- games, grammar explanations, giving you a text for homework and then reading it out loud and pouring over its during class. Our teacher liked to use blow-ups of our ID pictures he pasted on drawings on the white board. I don't know that I agree with all the Japanese ideas about language learning, but the instructors are committed and the texts and classes are good. Like all J-schools, it's pretty JLPT-centric, but I wasn't aiming for it and some of my classmates weren't either.
Also, the teachers are generally younger and are accustomed to being around foreigners-- they hear a lot of baby-steps Japanese all day and it's doesn't seem to faze them.
HOMEWORK
We had a TON of assigned homework, in my opinion, but the trick is that you kind of don't have to do it all. The open secret is that everyone's paying to be there, so it's not like you get detention. I couldn't keep up after a point, and I felt like a jerk not doing it, so I was kind of surprised when I got good marks for the semester. Part of it was that they drown you in xeroxes, and paperwork & I are not on speaking terms.
A lot of the handouts have stuff you won't find in any textbooks, too-- like m_m and ^_^; and other stuff-- I knew what [kana]kke[/kana] felt like, but not what it meant beforehand.
PRIVATES
You can do only privates, if you can afford it. It costs a multiple of the regular fee, but you would learn a LOT. I did a couple here & there, and found the instructors there (who are different than the other courses) of very high quality, and you have a lot of flexibility for what you want to study. It's Japan, so there's always a script-- no winging it. I ended up writing some essays with one, and then going over them. And there's just a lot more chance for conversation, even small talk, in a one-on-one environment. Definitely the way to go if you can afford it. I would love to go back and do this for a refresher for a week if I were going back.
MY LEVEL UP
They place you in a class according to past experience, a substantial written test, and a one-on-two interview with two teachers. I was rusty. But I had lots of fun during the interview (which tells you what the school's like), even though I'm all technique (rather than correct grammar). I felt way out of my depth in my class at first, but partially that was because of rust and not knowing how things worked. I spent the first two or three days frantically memorizing new classroom vocabulary ([kana]jidoushi/tadoushi nado[/kana]. Once I got up to speed I realized some of the other people were around my level, and I knew things they hadn't learned yet, so it evened out. I filled in a lot of gaps-- importing for self-studiers-- and feel way more comfortable feeling my way around in the language, and inferring meanings, which I don't think I ever could have come to on my own.
CRITICISMS
1. You'll hear a lot of English around the dorms.
2. The dorm cleaning check. Very paternalistic, very Japanese, very annoying.
3. You'll hear a lot of foreigner Japanese, too, which is good and bad. It's funny-- I had no trouble understanding American/German/Chinese Japanese, but some of my classmates who came from really different linguistic backgrounds and didn't have really good Japanese, I wondered what on earth they were talking about. The accents make it tougher, too-- Italian Japanese sounded like Italian to me, and I'm sure my Japanese wears a cowboy hat.
HIGHEST PRAISE OF ALL
Because it's a non-profit, fees are low, including the vending machines on campus! 100 yen, hotto coffee!