After trying out the service of Wifi-Korea for the first time during my previous trip, I chose to used their service when I visited Korea again together with my family. We stayed for a long 10 days, thus I have opted for Everywhere Speed Pack pocket wifi which cost only USD 3.99 per day since it covers everywhere in Korea.
You may select to pick up at their office at no extra cost, or pay USD $5 (each way) to enjoy the hassle free delivery service to your Korea accommodation doorstep.
Note that delivery service is only available if you book it at least 2 days in advance before your arrival.
Superbly convenient, all you have to do is to fill in the form on their web-page and make your reservation. Instead of picking up at the airport like before, I visited them at their office, chose to pay the deposit by cash (100,000 won) and got the device on the spot!