I see that there has not been much mention of Flybus in the recent postings and just wanted to put it out there that this is still a very easy and dependable mode of transportation from Keflavik to Reykjavik. You can purchase the ticket online but be aware that Internet Explorer does not work well for this site. I needed to make the purchase by accessing the site through Google Chrome as my browser. Our flight did not arrive until 11:40 PM and there was a bus waiting for us when we landed. Flybus offers an option to take you directly to your hotel called Flybus plus. For the hotel we were staying at we stayed on the bus but for some hotels there is a transfer to a smaller bus at the BSI. It is all orchestrated by Flybus so there is no guesswork involved. The trip from Keflavik is about an hour with the stops that are made along the way. Highly recommend this service for airport to city transportation.