This question, APS-C or FF, is really **THE** question that d-slr buyers need to ask themselves. Answering the question is up to each individual user. "Some" people find that BOTH types of cameras are useful to have at their disposal. Others are happy with one, or the other type of camera. Budget is a big issue; many cams are expensive! Lenses are expensive! Photography in the modern era is not a cheap hobby or profession. I honestly think that buying a good, clean USED camera is a viable strategy for the majority of shooters. That way, a better "class" of camera can be had, for less cash outlay, and the depreciation has already been suffered by the person who bought the camera showroom-new. For me, personally, I like the higher "class" of camera, but I hate to pay full retail for anything unless I absolutely MUST. SInce this is the CANON forum, I am assuming you mean, "How is the 6D, as a value proposition, versus the 7D or 60D?"
For the majority of "casual" shooting, and just "general photography", the APS-C cameras with good lenses are fine. There are scenarios though where the bigger format cameras have advantages. If you need to ask, I think honestly, the answer is APS-C is the better choice for the majority of "normal" photography, with most of one's money spent on lenses and accessories, like a top-grade electronic flash, a good polarizer, and some software products.