The marble surrounds in my bathroom need replacing. Are the fiberglass surrounds any easy fix?If so, are the glued in ones preferable to the ones nailed to the studs? Thoughts appreciated.

Corian or similar panels might offer the most long-term value. Do not glue them up with dark construction adhesive, though (don't ask how I know). Fiberglass tub kits are OK but they are not as easy to install as something with less flexibility. Clear silicone caulk is used. Painters tape and furring strips wedged into place should hold your panels fine as as glue sets up.

I have installed many Mustee fiberglass tub surrounds, best I've seen and they hold up well. Biggest job I've done was a 14 unit apartment complex. Putting them on top of tile.

Biggest thing is to get the surface as clean as you can, I used Fast Orange with an orbital sander on the tile, that was at least 15 years ago and they all look great still.