The first thing to note is that whereas you used to be able to get truly unlimited LTE data access from eMobile, that is no longer the case.  Since eMobile was purchased by softbank, they began enforcing a 10GB limit on existing contracts, after which the line gets throttles to an unusable 128kbps.

While 10GB seems small, it's still larger than the 7GB the other carriers (including Softbank itself) were offering at the time.  All new contracts with eMobile also come with the lower cap - so you may hope to be "grandfathered" into a 10GB plan.

On the other hand, Y!Mobile (as eMobile is now known) started offering a new unlimited service if you signed up for their new Pocket WiFi model, the 305ZT, which basically uses both Softbank's network and eMobile's network (LTE + AXGP).  This is a hokey situation because once you go over 7GB, the device will prompt you to purchase more data.  You must go through the purchase process even though the price is 0 Yen - and it's only valid for 2 years.

Recently Softbank came out with a different (read: more expensive) contract where you can have unlimited data for more than two years, by paying more and only using the AXGP network.

But here's the real rub, whether you use the "buy more data for free" method, or you sign up for the AXGP only plan, you have no monthly cap - but you still have a daily cap!  To be more precise, you still have a three day cap.  Basically, if you go over 1GB in three days you get capped.

That means, if you go over 333 MB per day, your access will be throttled even if you are on the "unlimited" plan.  This makes eMobile about useless for watching internet TV, lots of Youtube, backing up your computer, or even downloading Windows updates.

Yet sadly, it's still a better deal than the other LTE operators if you are interested in the high end of the market.

If you come to the realization that anyway your internet will suck if it's mobile and so you should get a land-line, then you may suddenly become more interested in the low end of the market.  This is where Japan has seen innovation lately - the MNVO area.

For example, OCN has plans ranging from 900 yen (less than $10 US) for a data only SIM over 100MB of data per day - enough to use mail, web, etc. on a smart phone, to plans with 5GB of data and voice service and still cheaper than normal carriers.  Honestly 7GB or 5GB isn't much difference, and the OCN plan will cost you much less than a similar plan from DoCoMo, while offering the same speed and coverage.  In fact, the main difference between MVNO plans and carrier plans at this point is that MNVO plans have high (un-discounted) per minute rates for outgoing calls.  Another good thing about OCN is that even when you are throttled, you still get over 200 kbps.  That doesn't sound like much, but twice as fast as what you'll get under eMobile, and still fast enough to use for VOIP.

OCN isn't the only MVNO, and in fact Mineo is interesting in that they support both DoCoMo and Au infrastructure.

The MVNO players also now offer same day MNP where they can hand you a SIM card at a limited selection of shops instead of mailing to you, as well as a selection of handsets to choose from if you don't want to bring your own.  Both So-net and OCN offer a variety of handsets to choose from.  

Even though few of the MNVO players offer high end plans to match DoCoMo, etc., they are actually not bad deals.  For example, you can get 5GB for 1500 JPY, which works out to 10GB for 3000 JPY, so it isn't unreasonable to simply get two SIM cards in some cases.  

Some MVNO operators also have other interesting middle range deals.  For example, you can get a SIM which offers only 500Kbps speed, but for up to 15GB per month, and you can get SIM cards with daily limits instead of monthly.  that means if you go over your limit and get throttled today, you will be back to full speed in the morning.

On OCN's daily plans, data not used on day rolls over to the next, and you can pay 500 JPY for unlimited data for one day.  On the monthly plans, unused data also rolls over.

Another company called ServersMan offers ultra cheap SIM cards with very low speeds (250Kbps), for very cheap.  this would be perfect for something like a vending machine, cash register, or other device that only needs to communicate status and receive commands.  

On the opposite end of the spectrum, there are a few higher end plans offering 8 or 10 GB with no annoying limits.  For example, wireless gate offers 8GB per month with no daily (or 3 day) limits for 2297 yen per month.  That's a better deal than you can get out of any of the MNOs for sure.  On the "Unlimited" side of the spectrum, you can get unlimited use at 500kbps from UQ Mobile for 1680 jpy, or 3 Mbps from NTT Pulala for 2760 JPY.  3 Mbps actually isn't bad at all, as that is more than enough to use the connection for watching video.

Besides the usual suspects like OCN and So-Net, there is also Nifmo, BIGLOBE, fretel, Pulala, DMM, b-mobile, and a large number of other MVNOs out there.  Recently, Panasonic also got into the MVNO business as well, with their "Wonderlink" SIM cards.  (Which of course are Docomo).

The point is, there is a lot of innovation going on in the MVNO area, and since many of the companies don't require a long contract, there's room for experimentation.

Speaking of smartphone plans, all of the major carriers have gotten wise to the fact that everyone is using LINE and similar services, so now nobody offers unlimited data (which was a lie anyway), and instead clearly offers a certain amount of data (usually 7GB), and unlimited voice calling

Lastly, for people who still want to keep heavy usage while on the go, WiMax is still around.  A while back, there were worries that the two year "unlimited data" promise would expire, and everyone would suddenly have 7GB caps just like LTE.  Thankfully, that hasn't happened.  In fact, UQ Communications has unveiled a new plan (which is slightly more expensive), under which WiMax 2+ is unlimited.  In either case, they have three day cap of 3GB, after which you will be throttled - still, this is 3 times the cap of eMobile!  If you do the math, you can only use 10GB in a month on eMobile without being throttled - assuming you are on an "unlimited" plan.  With wiMax, you can effectively use 30GB in a month without being throttled.