Sanzan comments

With MVNO's, you get what you pay for. The monthly rates are low, and the contract terms fairly reasonable (in general, you cancel the service without penalty after four months). But the data-transmission speeds are pathetic. The best I've seen on b-mobile is 300 kbps, intentionally crippled by NTT. I doubt Rakuten will fare much better.

There is plenty of info in Japanese about mvno services and their data speeds. Bmobile is just the one that gaijin have used because they sell services in English. The major mvno providers on docomo, Biglobe, Iijmio, OCN (err, part of NTT themselves) have a rep for achieving speeds up to 10Mbps, with the caveat that they can be slow at peak times like lunchtime on a weekday. I've been getting 4-5Mbps off Umobile since they upgraded their service last week. There is a list of sims on kakaku.com and info on sims and compatibility with different handsets can be found on sites like mvno-navi.com

If anyone out there is worried about data speeds off an mvno, buy a 1GB prepaid sim off the ones that sound good and stick it in any docomo compatible phone or tablet. Test three or four of them with plans that suit your needs. Iijimio has lots of resellers, so you could just go into BIC camera and ask them to show you a speed test in the shop. Most mvnos will give you a phone number (MNP okay) and 3GB a month for under 2,500 yen.