EDIT: added perspective on the bandwidth cap.
I just read through the TOS for the b-mobileSIM U300. Our anonymous source was correct about no streaming media, such as videos or VOIP. In addition there are also bandwidth caps. B-mobile will also collect and analyze traffic data, i.e. (deep?) packet inspection, in order to determine when to cap a user's bandwidth. (Though I haven't checked, I am pretty sure all carriers do this - people tend to freak out though when an ISP does it.)
Here is the link to the b-mobileSIM U300 page. Following the green 購入する button in the upper left gets you to the signup page:
There are two options for payment, credit card and COD. Before signing up, you have to agree to the terms and conditions. I'll paste some of the important conditions below
From the time of first connection, the product is valid for 365/185/30 days depending on the subscribed plan.
有効期限を経過した場合は、本商品を利用することはできません。利用時間を延長するには、別途チャージしていただく必要があります。（有効期間終了後60日を経過するとチャージできなくなります。）This means that if you get 30 days of service, let it expire, then renew before 60 days have passed, you can continue using the same SIM. After 60 days, you'll need to re-register and wait for a new SIM to be delivered.
The product cannot be used beyond this time. An additional charge is required to extend service. (Service cannot be extended after 60 days have passed from service expiration.)
他社プロバイダやリモートアクセスサーバへの接続はできません。I guess this means they won't allow VPN pass through, which makes sense if they want to inspect packets.
You cannot connect to other providers or "remote access servers."
Below are the most important bits.
本商品は、テキスト・静止画像等のweb閲覧やメール送受信をモバイルで快適にご利用いただくための商品です。そのため、動画再生やファイル交換(P2P)アプリケーション等、連続したデータ通信にはトラフィック制御を設けています。As discussed previously, this was pretty much inevitable.
This product is designed for activities such as browsing of text and still images or sending and receiving email. Therefore the streaming of videos or the use of P2P application, etc. will result in bandwith limitation/traffic shaping/network management/your @$$ being throttled.
特にご利用の多いお客様（当日を含む直近3日間のパケット通信量が300万パケット以上）は、それ以外のお客様より通信の速度を制限する場合があります。First, how many bytes is in a freaking "packet?" I absolutely hate the use of the word packet because it is just a way to get around saying up front how much bandwidth costs. Rant aside, I believe 1 packet is 128 bytes.
Customers consuming in excess of 3,000,000 packets within three days may experience additional limitations on transfer rate.
So, 128 bytes * 3,000,000 packets = 384,000,000 bytes.
384,000,000 bytes / 1024 = 375,000 Kilobytes.
375,000 Kilobytes / 1024 ≅ 366 Megabytes.
366 Megabytes / 3 days ≅ 122 MB/day.
Exceeding around 100 MB/day for 3 consecutive days will get you temporarily throttled.
On the train this morning, I pulled out the back of the envelope and calculated how long you would need to sustain a transfer at the absolute maximum of 300 kbps to hit the cap. In short, it works out to an hour a day for three days.
In doing this, I realized that a packet kind of makes since. One packet is exactly equal to one kilobit.
1 packet * 128 bytes/packet * 8 bits/byte = 1024 bits = 1 kilobit
1,000,000 kb / 300 kbps ≅ 3333 seconds ≅ 55 minutes
But we already know that a sustained stream will activate the rate cap after "a few minutes," so in reality, you would need to do a number of burst transfers each day to hit the average daily maximum of 122 MB. If the cap kicks in after 2 to 3 minutes of streaming, you'd need to do around 20 full-speed bursts per day.
I am not a heavy user of packets because I am typically connected to wifi, don't watch youtube videos, and don't have a VOIP app installed. As a result, I typically don't even use half of the 3 million packets mentioned above in an entire month.
迷惑メール対策としてOutbound Port25 Blockingを実施しています。TCP port 25 is the default SMTP port. A few years ago in Japan, and not so many more before that, very few admins forced the use of SSL in mail, which typically uses port 465. Now, most servers I see not only allow SSL but require it. So, this shouldn't be a problem. My guess is that this is to prevent people from sending spam through open relays, though to be honest, I don't think there are so many open relays these days...
To prevent spam, TCP(?) port 25 is blocked.
Internet connections may be temporarily disconnected
弊社は、上記の利用条件で本商品を提供する為にトラフィック情報の収集、分析等を行います。Packet inspection. How deep? I should also mention that the Japanese phrasing does not limit the word "analyze." It says analyze, etc. 分析等（ぶんせきとう/ぶんせき など）.
To ensure compliance with the above conditions, we will collect and analyze traffic data
This service is provided as "best effort," and the advertised speeds are not guaranteed. In addition, depending on location and signal strength, actual speeds may be reduced.
Service may not be available in locations removed from base stations or where structures may interfere with the signal.
The provided SIM card remains the property of b-mobile and must be returned upon termination of services.