Is it worth it to replace hollow-core interior doors with solid core doors?
The differences between the types of doors is pretty obvious in use:
SOLID CORE DOORS

Mimics the appearance and feel of a solid wood door
Muffles sound better
Heavier and slams better

HOLLOW CORE DOORS

Lighter and easier to handle than a real wood door
Cheaper?

Jeld-wen says:

Hollow core doors are a good choice for areas where sound transmission is less important, like a closet or pantry.
Solid Core doors are heavier and more resistant to normal wear. Solid core doors are commonly used in areas where more privacy is preferred. If interior noise reduction is a priority, consider the JELD-WEN® ProCore The Quiet Door®, which reduces sound transmission by up to 50 percent when compared to a hollow flush door.