, a reader with a the B-mobile USIM 300 data-only SIM card has been using the product for a few weeks now and is satisfied it's performance. He used the speedtest.net app over a few days traveling between Kyoto and Fukuoka, and the results so far indicate he is getting the advertised speeds. Obviously, more testing is needed, though. But, so far so good. Click the above image to enlarge. Click here for related posts.
My general feeling after two weeks (I received the sim on 25 April, 3 days after order) is positive. The speed limit does not affect the experience for applications I'm using the most: Gmail, google chat, Facebook or twitter are working great and speed is not an issue (uploading pics from the Facebook app is reasonably fast). Even Google map application is working good. Of course the maps are downloading a bit slowly but if you are using Directions (with android 2.1), you are very likely to stay in the same area or to update infrequently.
Youtube is actually working, but it needs a very long buffering time before the video starts. Do not expect much on this side.
Browsing is a bit frustrating because of the speed limit, and probably because the speed is not constant. The pages appear quite fast, but it usually takes a long time to finish downloading. Nevertheless, it is still useful to get a full browsing experience (btw, skyfire beta is great) and it does the job if you're patient.
Market is working and downloading an application of 3Mo is no problem (takes about a minute).
Docomo's network coverage is really good. I am using softbank (prepaid card) and I now understand this smile on my docomo-using friends faces when I bitch about the "no signal" message when having dinner in an underground izakaya. It's actually so good that I could use it in the shinkansen from Fukuoka to Kyoto (not all the way of course, but still).
Final thoughts, for me the price is right for the service. It's half what you would pay for softbank's packet plan and it works for most of the scenarios I need. The only down-side of this b-mobile sim is the fact that it cannot register on the network to receive calls. So I still need to carry my old softbank prepaid cell and this is why I will probably not stick to this option but only activate it from time to time.