if you hate the feeling of opening and closing cheap doors, then you should do the upgrade. (i hate eating with a cheap fork, so i understand.)

if 'worth it' means will i get my money back on it, it depends on the fit and finish of the rest of your house. but, probably not. make yourself happy.

We always suggest to our clients to put their money into the details that you are hands on with everyday, and therefore interact with the most. We made the upgrade to our own doors when we rehabbed our historic home ten years ago. Well worth the money just having something feel solid day in and day out.

I prefer solid over hollow. I recently sold my condo in Brooklyn where I've installed solid wood doors few years prior. I cannot positively say if any of the prospective buyers noticed on their own but I did point it out to every person and knocked on wood. My reference to solid wood doors also indicated that even minor things in the apartment were replaced with premium upgrades.

Not sure what you are seeing as the cost difference. We're seeing roughly $50 per door (assuming split jamb) difference in SC vs HC - so that can mount up in a whole house. If you do it, do it because you want it. We haven't seen many clients that have any idea what the difference is and I doubt would pay for the difference in resale.

The best way to soundproof a door. Don't forget weatherstripping to keep the noise out (or in).