WiFi "eggs" (at least that's what they call the pocket wifi in Korea) seem to be quite popular now. Back in the day i used a data sim from B-Mobile and didn't have any complaints. This time I picked up a data sim from so-net (at a BIC or Yodobashi Camera) and although the service is great, the data is limited and refills are frustratingly expensive (¥5,000 for the first 3GB, but the refills are only up to 1GB@¥3,000, so it adds up fast). Seems that the pocket wifi are the way to go, especially if you want to connect with multiple devices.