I had a 630. That keybed was the same as my PSR3000 and I've seen no evidence to suggest the later PSRs are any different.

I'm not sure build quality has deteriorated as such but the newer PSRs are deeper and heavier which will make the entire thing flex more than the older model so it will feel less solid.

Sounds - I didn't replace my 630 with a 2000 because I thought is sounded bad, I wanted the newer display and operating system. Then I changed the 2000 for a 3000 to get on board storage and the improved song management (i.e. the "next song" function). The megavoices were a bonus.

I've got an old MU80 which still sounds pretty good to me,

I never had an issue with the 630 display when I had it, but as soon as the 2000 came out with the full-screen lyric option I thought "that's the one for me"!