Just received the PL-660 and listening to it now. Unfortunately, lent my 909X out to a friend, so can't put them side by side.
But, what I can say from using the PL-660 and my memory of the 909X:
The PL-660 has pretty decent reception on the lower HF bands with the whip. The 909X was sorta deaf there w/o an ext antenna. Dark out, so haven't tried the upper bands yet. Was able to get Radio Ronin Shortwave on 6925 tonight with the whip and sounded alright, about S5. Perseus with 102' G5RV was about S8. (no, not comparing the 660 with the Perseus, but giving reference).
Audio quality is decent for a small speaker. 909X sounds better, but PL-660 is far better than the tinny Sony ICF-SW7600GR.
MW seems to be pretty good, picks up plenty of DX. LW is ok, but my Sony ICF-2010 beats it by a little bit. Not a big surprise there. Listened to NDB AA in Fargo on 365 kHz (about 200 mi away). It was just barely coming in on the 2010 at the time and the 660 didn't hear it. Waited a couple of minutes and it was better on the 2010 and the 660 was starting to pick it up.
Sync works as advertised.
Overall from initial impressions, I'm very pleased with the $120 radio.
When I get the 909X back, I may line 'em all up (the 2010, 909X, 660, and 7600GR) and see how they all compare side by side.
So far, about the only thing I don't like about the 660 is that it doesn't have line out.