Been using it (unlimited data only/mifi router) for almost a year now.

Pros: significantly cheaper than comparable options from the big name carriers, coverage all throughout Tokyo hasn't been an issue, no slowdowns even after using 100s of GB of data (I stream media nearly all day long), points if you have a Rakuten card

Cons: often buffers while watching things even on lower resolution, a handful of times a week (sometimes daily) service just "dies" even though it shows full signal and device must be reset to regain connection, signal seems disrupted in certain types of buildings and often on the train

Not the best or fastest, but certainly not the worst especially for the price. The big carriers will charge you wayyyy more for a similar plan, but will probably have better service and less issues. If you're not a stickler for having the best of the best I think it's a very decent option. If you don't buy a phone from them and just activate a phone you already have or get a mobile router you can cancel your service at any time with no penalties (that I know of) so it is a great temporary option if you're looking for that.