It almost looks brand new! Very worth the effort. This is not a difficult job, just tedious. It requires you to plan to be out of your shower a week to do each stage correctly.
1- Acid wash floors (or use a grout de-hazer)
2- Remove caulk
3- Stain the grout. I found with small tile it helped to use small, damp squares of an old tee-shirt to wipe the stain off the tile because anything bigger dragged spreading more stain or removed it from the grout.
4- Apply new caulk. I bought this tool and it worked like a charm!
5- Seal grout as many times as necessary so I won't have to do this again any time soon!