I have flown them several times from BWI/NYC to Guadeloupe/Martinique. I've had a great experience every time, both at check-in where they are almost ridiculously helpful to the flight itself which is very pleasant in new planes that seem to have good seat pitch. And that's before you take into account that I generally end up paying something on the order of $200 for a direct non-stop to the Caribbean from Northeast in the middle of winter when everyone else charges min of $500 with at least one stop.
I haven't flown them to Europe, but based on my experience I would choose them over a legacy U.S. carrier in coach any day, even with elite status on several of said legacy carriers. I'd say they're way more like a JetBlue or Virgin America than a Spirit which is what I'd equate WOW to.