Wow airlines has some very low prices, which become somewhat low if you have to pay for baggage. The biggest drawback that I can see so far with them is that their layovers to some cities in Europe from the West Coast add to a total travel time of 28 hours, due to the long 15 hour layovers in Iceland. Such a long time basically means another day is devoted to travel and not to the trip itself and starts to eat into the whole idea of choosing a budget airline in the first place. Otoh, while San Francisco to Berlin is 28 hours, San Francisco to Amsterdam is only 13 hours due to a much shorter layover.
There's always some kind of drawback to a "great" price. I have chosen layovers in some instances, but only those that add value to my trip (using Turkish Air as an example, I don't just want to sleep in Istanbul, but actually have time to see something I want to see...so the exact layover timeframe is important to me). I did an Iceland layover once (5-6 days) and loved it, but it was a budget buster. I think you have to be clear on your "real" intent of flying a budget airline - if it's because you want to save money on the whole trip, that's one thing. If you want to see another country as part of a layover, that's another issue. The latter can either lengthen your trip, or cause you to shave off time from your original trip, and/or end up costing you more in the end. It is still cheaper than two separate flights on different dates to see both countries (your ultimate destination country and the layover country), but yes, you should be prepared to pay more (in time or money) if you add another place into the mix because it's advertised as a "free" layover.
We just flew WOW and it was great. We had a 2 hr layover in Reykjavik. Their prices are super low. Even if you have to check a bag, it's only $50, which still brings you in less than others.
Most/all of WOWs flight lengths are as reasonable as all other airlines. You have to be careful with length of time on any airline, WOW is no different.
"...SFO to Berlin is 28 hours." ...lovely. One can also fly Air Berlin from SFO with a layover at LAX then direct to Berlin making it 11 plus hours.
A good tip-off to look for an extended layover is if one of several National carriers come up, IcelandAir, Aer Lingus, and Turkish Air are all famous for low prices that typically come with a long layover in-country. The tickets are essentially subsidized by the government in many cases with the hopes you will turn that layover into a few days. For IcelandAir, it works, many people, even on this board, have stayed in Iceland where previously they thought "maybe someday" but might not get there. There also used to be some deals with Eastern European airlines (LOT and Czech Airlines) but I have not seen those in a while.
My opinion is that the long layover is fine if you have enough time to see a few sights or can extend it to one or two nights for the same deal and it works into your itinerary, otherwise, spending 10-15 hours waiting for a plane is not worth saving a couple hundred dollars
I just learned that Finn Air also has a stop-over program (with some caveats):