I’ve only tried the ones going south (SJ to Malmö/Copenhagen and SJ and Snälltåget to Hamburg and Berlin) and have found them to be perfectly adequate. The rolling stock is old (ex-Deutsche Bahn) but refurbished, they’re mostly punctual (though running through Germany can compromise that) and offer a comfortable journey. SJ Stockholm to Berlin (leaving at around 17:30 and arriving before 9 the following day) is just about manageable, though once the Fehmarn Belt tunnel is complete, I imagine that will depart around 20:00 to arrive at the same time. Other than shortening the journey, my wishlist would be ÖBB NightJet-style single-person capsule compartments, onboard WiFi and a dining carriage (though apparently they’re working on the latter).