Idol cards continue to be a big thing in Japan, but I gather that they had their peak about 10 years ago. And just like any other trading card release, the hits are the big draws and the justification for high pack prices. Autographs and relic cards are common, though these card sets also can include kiss cards and polaroids. "Cheki" cards are quite common in idol sets, and even the women-in-sports set Real Venus and the recent JWBL release by Epoch included chekis. On the less-rare side of the spectrum, in addition to regular base cards, most releases include SP cards as part of the regular set or as separately-numbered insert sets and foil or facsimile autographs.
Yuko Ogura is my focus for idol cards, and I've gathered a few rare cards of hers. I have a framed photo (a premium given to purchasers of cases or large numbers of boxes), autographs, and a cheki. And of course some costume cards, like the newest addition below.