They are fantastic. I succumbed after reading an article in The Times a few weeks ago about how they are chic and look fabulous under all sorts of things including wrap dresses etc however old / big you are! I've been living in them for weeks under shirt dresses, smock dresses, wrap dresses etc.
Personally, being shortish and a classic english pear shape I favour full length rather than cropped as they elongate the leg and are more flattering to my shape. I've had loads of compliments when I've teamed them with wrap / shirt dresses - a couple of friends raced out to buy themselves some too! I'm 40 and don't have great legs so they are a lifesaver if you don't want to wear tights and/or high heels which was the most flattering way for me to wear dresses, until i discovered leggings as i can now wear dresses with leggings and flats.
I've looked at American Apparel, Top Shop, Falke and Tchibo. M&S do good ones too. All good but different (fabrics, price etc - some do different denier like Tchibo).