I think bodice ripper specifically refers to historical romances, although their quality varies. I read a lot of them as a tween and early teen, and I have to agree with Reinadeluz that they can teach you about or at least spark an interest in a certain time period. I don't know how popular they are any more because I am not a romance reader at all any more. I never see one for sale any more though.
I discovered a hidden cache of them (this was the one genre my father disapproved of, not because of the sex, but because he felt I was wasting too much time reading them) when I was packing for uni, and I re-read a couple. Some were really bad, but I remember there were like three writers who didn't adhere to the more formulaic method of writing them and had heroines who had sex outside of marriage and didn't end up marrying the men who "deflowered" them.
The formula, for those who have never read them is:
Virgin meets future husband and hates him. They eventually end up in a situation where they end up having sex, usually slightly against the heroine's will until her body defies her and gives into the seduction (hence the nickname). There may be some other complicating factor like another betrothed (typical), a desire to stay single, kidnapping, or villain other than a former love interest standing in the way. The heroine usually discovers that she's pregnant. The couple may meet a few times for clandestine sex before the wedding. There should be 3-4 instances of getting it on in the book, a few of them rather graphic in that romance sort of way with the quivering, gasping, and manhoods alight. Anyway, the couple end up married, and depending on the length of the marriage conflict, the novel may soon end, but sometimes there are other things that try to break the couple up. Eventually everything gets resolved. The villains get punished or find redemption. And always, everyone lives happily ever after unless it's a series. THE END.
I did pick up an interest in history reading them. It's not the same level of information as you get from reading a non-fiction account of the era or even historic novel, but it can give you background. Sometimes the detail these writers go into is a bit surprising. But even with all that history, I could never see myself reading them again unless it was someone who didn't follow the formulas.