They can be really great. I've gotten them many times and taken advantage of them. I also offer them when i have a few of the same item, and the other offer is acceptable to me.

Ive sent a few offers that I was really surprised the buyer didnt snatch up. I think some buyers just aren't sure what it is and dont take advantage of a good thing.

But...as Steve said, always make sure it's legit...there are some sharks out there waiting to prey on people that get sloppy. You can make sure it's in your "my eBay" as an offer. If it is...it's legit.