What an incredible experience! We saw three different humpback whales on our tour, and all of them were close enough to get some incredible views. The boat ride itself was very pleasant, with serene views of the Atlantic even when the whales were not around. It did take a while to find them, though we enjoyed drinks in the lounge in the meantime.
This tour is LONG, so be prepared to go to bed a bit late. There isn't likely to be anything open afterward so eat beforehand (the boat sometimes serves soup and sandwiches, but ours was out). I would also recommend coming prepared for the cold. Even though the weather on land was mild, out on the ocean it was freezing! They do offer some waterproof overalls to borrow on the boat if you'd like.
I'm really glad we chose to do this tour with Elding, who are respectable contributors to the research and conservation of Iceland's whales. It was clear to us that Elding was dedicated to the well-being of the animals we were visiting, and that they went out of their way not to disturb the whales in the process.
By the way, if you don't want to be bogged down trying to get photos yourself during the tour (and it's really hard to do - the whales are fast!), the tour guide will send you the professional photos afterward on request. I really recommend emailing about it. We got some beautiful pictures of the whales for friends and family without missing a moment ourselves!