I like
JOI, because I've had good experiences with them. You can do an intro of 3 lessons for 900 yen, and take them pretty much whenever you want. They cover levels from N5-N1, in speaking, grammar, and vocab. (Separate classes for each, usually.) Yeah, their website looks a little cheesy, but I've had nothing but good experiences with them so far.
The pricing structure depends on a few things: if you're a dollar person, you can pay in dollars, but I find it's cheaper to pay in yen, because the yen is really weak these days, and they don't update the payment site quickly enough to reflect the changes.
Kind of like an arcade, you buy packages of lesson "tickets." You can buy as many or as few as you want.
I usually pay around $200 or so for 30 group lessons. That's $6.67 a lesson. For the JLPT, I'm burning through 5 a week, but usually I might do only 1 or 2 a week. Also, depending on the package, tickets have a fixed expiration date.
The lessons are generally 50 minute group lessons, with 2-6 people there. They are set up to be independent "chunks" so you can come and go as you please. (Extremely useful, but lacks the review ability... but I get why. It's more convenient for everyone that way.)
Classes are all in Japanese. (At least they are at N1 level. Dunno about other levels.) You can check ahead of time to see what topics the teacher plans to cover, so you can select classes that will do you the most good.
The classes are done in a Java chatroom program that they run when you log in. The teacher will start up a webcam, but you don't have to watch it if you don't have bandwidth. It's mostly voice/text chat, plus she'll use a Powerpoint-like whiteboard to present materials. Afterwards, you can save the chat window (which some teachers will put all of the relevant notes into), and/or the teacher will send you a lesson PDF. Varies by teacher. Generally, it's like a moderated chat. Keep your mic off until it's your turn to talk, otherwise it's chaos with feedback and echoes.
In grammar classes, the teacher will usually cover two topics at the level you want to study. Usually, a basic introduction to the topic (meaning, formation), some examples, some questions, then come up with your own sentences, and she'll correct them. It's a good way to "get" a grammar bit you don't know.
If you have the Kanzen Master books for whatever level you want to study, then you have all the textbook you need. (You don't really need one, but it's handy to have if you want to follow the teacher.) They have an agreement with 3-A, the publishers of KZM, so that's reassuring, too. 3-A makes good stuff!
The kanji/vocab classes are also really useful, because sometimes they'll cover stuff I never bothered to learn, or just never got to. I've done a few conversation classes, but I already have a conversation partner, so I don't really use those. I might use those after the N1. The ones I've taken have been good, though.
They also have a weekly N1 exam course, but it's almost over. They usually do it for the December N1, and it's really good, IMO. It finds a lot of my weak points. Argh.
You can also do private lessons, too, but I've never done those.