I never even really wanted diamond stud earrings...and then my husband gave me a pair for Christmas years ago. At first I wore them from time to time, but grew to really love that they completed every outfit, no matter how dressy or casual. I don't know what size mine are, but I think they're just right -- not so small as to seem insignificant, not so large as to seem ostentatious.
So I do think they're worth it. I have an old pair of CZ earrings that are smaller stones, set in a silver setting, and they were good enough for me for a long time, but once I had the diamond ones, the CZ's hardly ever got worn.