You know, there is a lot of Promise Ring Bashing here on PS and I really don't get why it bothers so many people. It's really not a big deal. It may not be your thing, but I don't really see why so many people are offended at a girl wearing a ring that isn't an engagement ring on her ring finger.
I wore one for probably 8-9 years! It was a thin yellow gold band (bent out of round from wear) with the fugliest brown frozen spit cheapest diamond on earth. But at the time I thought it was very appropriate for our age/status in life. I had to have been 18-19 years old, which would have been too young for us to get engaged. I thought it was a sweet gesture for my DH (then boyfriend) to shell out the $100 or whatever as a nice gift to show me. He was probably making minimum wage at the time and we were both in college. But he knew we would marry some day (and he was right) and it was just a gesture to show it. You don't have to marry or get engaged to be committed, and it's really just a simple way to show commitment. I actually didn't really care for yellow gold, but I wore it until he gave me my engagement ring. I had a few people think it was an engagement ring, so what? They thought I had a $100 ugly-diamond engagement ring... So? Why would I care what they thought? I had no problem explaining "no it's just a promise ring, we are still in college..."
It is interesting to me when we get guys in their late 20's or older saying they want to spend big $$$ on a fancy promise ring for their girlfriend. Yeah, I think that situation is not quite the same as a teenage couple, but if that's what they want to do.... Who am I to judge?
ETA- Actually I got it 8-9 years ago but I was engaged 4 years ago so I only actually wore it for 4-5 years. Oops.