While the feature set of the S544 is somewhat reduced from its predecessor, the most important aspect, its performance, is every bit as good. As long as the source files are of a reasonable quality (think iPlayer downloads, not YouTube rips) then videos look pretty snazzy. Colours are vibrant, the screen is bright, playback is smooth and clear, plus viewing angles are good enough that two could watch a video at the same time. I'll leave it to your imagination as to when that might be useful.
Audio performance using a decent set of 'phones is every bit as good as the S639F's. Yet again there's plenty of warmth, clarity and detailing that the likes of an iPod simply can't match. The Offspring's typically crashing guitars and catchy lyrics on Americana are reproduced with plenty of presence, great low down detail and a generally catchy tone. The likes of HeartsRevolution's synth-ridden CYOA! are portrayed well, too, the punchy output of the S544 making it all the more addictive listening than on a lesser player.
It should be said that some players like Cowon's iAudio S9 still sound better, especially with more sophisticated music, such as Wagner's Tristan and Isolde. However, the difference is only really noticeable with a fairly high-end set of headphones in a quiet environment. Plus the S9 is a fair bit more expensive, which is an important consideration.
Ok, so audio performance isn't quite up there with the best players, but can it outshine them with its format support? Well no, not really, as that's the S544's real Achilles' heel. Even though for most, MP3, AAC, WMA and Linear PCM support will be sufficient, when rival players can also play back OGG and FLAC files it's annoying that Sony's player does not. If you can accept that limitation, though, you should find the S544 a perfectly capable player.
The Sony NWZ-S544 doesn't quite impress as much as the NWZ-S639F it succeeds, the NWZ-E443 sounds almost as good while costing less and there are better sounding, better featured players out there. But accept its shortcomings and the S544 is still a credible player.