If you want internet service and/or cable TV in Japan, this is the best bet. It's no more expensive than any of the other options (in fact, if you live in a big building in a big city, it may be MUCH cheaper), it includes phone service at no extra charge, and it's by far the fastest service available. You can get a 100 megabit fiber optic line right into your apartment.
First, DON'T get a phone line. A phone line plus DSL service will run you about 6000 yen/month (plus calls). However, for 5670 yen a month, you can get a 100 Mb/S fiber optic line right into your house that includes both phone service and super high-speed internet (if your building is already wired for the service, it may be as low as 3035 or 3570 yen under the Mansion Plan, depending on how many people in your building have the service). Speed? When I download something that's got a lot of seeds, like The Daily Show or the latest episode of Battlestar Galactica, I'm generally able to download the entire show in 3-6 minutes (that's for 360 MB!); if I've got a couple of downloads running, it's not unusual for my total download speed to several MB/s. Service has actually been getting much faster lately. You've got two options for phone service. The included phone service (no extra money) works just like a regular phone (including quality) except that you get a number beginning with 050 and you can't call the emergency and toll free numbers. The deluxe option is to pay the phone company another 525 yen a month for Hikari Denwa service. This will give you service exactly like regular phone service--you have a regular Fukui prefix and you can call the free and emergency numbers. In either case, you use a regular phone and you get discount rates (e.g., 9.4 yen/minute to US). You DON'T need a phone line for these services--if you get one, you won't use it--it'll be a huge waste of money. If you got a phone line for free (some JETs do), this will replace it (you can keep the same number if you get Hikari Denwa); you can cancel the regular phone line and then you don't have to pay the 2500-3000 yen/month phone bills (not including calls!). If you call AsahiNET during one of their frequent campaigns, installation is free and you'll get up to four months free (they said the same thing when I signed up and I got seven months free--not sure why but I'm not complaining).
Details: The company that offers the best service (in ENGLISH!) is asahiNET. There's a number on the top left of the page to call and check availability at your address. They handle all the details and arrange installation with the phone company (NTT or TEPCO)--the only billing you'll get from the phone company is the 525 yen if you get Hikari Denwa service. If you get Hikari Denwa, you want the Kihon Plan. Unless it's changed in the year since I got my service, you call AsahiNET, and if service is available for your location, they arrange for the phone company to call you so you can arrange a time to be in your apartment to let them in twice--once to inspect your place to see what's necessary to install the line, and once to actually install it. If you don't speak Japanese, then when you call asahiNET, ask them to have the phone company call your supervisor instead, as the the phone company people don't speak English. The phone company will deal with all the stuff with your landlord about getting permission for the installation--you don't have to do that (and shouldn't try--it'll just create confusion). The only thing you'll ever have to deal with the phone company for is letting them in (have your phone and your mail from AsahiNET handy and your computer set up when they come so they can make sure it works before they leave). All customer service is in English through AsahiNET. When I set it up, the customer service people I talked to were native English speakers and knew, amazingly, the answers to all my questions.
You can also get cable TV over your fiber optic line, too. For another 2100 yen/month you can get either all the channels or unlimited videos; for 3150 yen you can get the channels and the videos. There's no installation charge. One issue to check is language. On normal cable TV systems, foreign movies and channels are bilingual--you can hit the language button on your remote or in your TV's menus (all modern TVs have this function) to get either the original audio or the Japanese-dubbed version, so you can watch stuff (including the Japanese versions of channels, like CNNj) in English. Since this service connects to a normal TV through a normal-looking set-top box, I'd imagine they haven't deleted this feature, but you should ask about it when you call AsahiNET.
One more thing. If you do sign up, please tell them on the application form that I referred you (the Introducer Campaign). My user ID to enter there is 
. Thank you very much.
If you have any questions, email me.