I have good friends from Norway who just visited the US and flew Iceland Express OSL-KEF-EWR. They commented the fare was a big attraction for peak summer travel, and that the service was very good. Crews were very friendly, aircraft were comfortable and good choices for their buy-on-board food and beverage service. They apparently paid a fee for seat assignments and bag check as well, but overall felt they got good service for the price.
Their only negative comment was the weather in KEF was not good and flight quite bumpy for awhile. They just flew back last weekend uneventfully and stated they'd fly them again......"better than Icelandair." These are folks who also fly SK via CPH so it appears this new transatlantic LCC is doing a nice job.
Good Luck with your trip.
watch what you want. you may get it.