From what I've heard, it takes a lot of eggshells. A snail or slug can crawl over a few shells, as well as around them. You have to have enough shells for them to get the message that the area is not pleasant.

Also, make sure the shells are cleaned or they may attract other animals.

Yep, I had the little buggers eating my hostas. I put enough egg shells near the plant so that they could not crawl in between the pieces.
It does help.

I was having problems with slugs/snails in my lettuce bed. I put a "band" of crushed egg shells around the perimeter of the plants, and I've seen no more sign of slugs/snails since.
The change has been so drastic, that I wonder if some other cultural/environmetal factor has contributed to the decline in the slug population. For now, I will assume that the egg shells did the job.

I haven't had any real issues with slugs in my Strawberry beds as of yet. But, I will try some eggshells there if needed.

I have also heard of some folks mixing wood ashes and crushed egg shells, and spreading that... but I have not tried that yet. Over time, that should make a nice contribution of calcium and potassium to the soil.