These games are brilliant, old though they may be. Even though there are the free remakes, the originals still have their charms, such as the old EGA style graphics or the typed command interface. I really recommend buying these games, however if you do, I also recommend using ScummVM for improved customisation in the games and easier fullscreen and other things.
I haven't actually played them yet, but I have seen a brilliant let's play on them at https://www.youtube.com/channel/UClIzxxkJ1GexBd6GwyacNGw, no this is not my channel. Anyway, I think that these games (bar one of them) are brilliant. In particular King's quest 6, which has by far the best writing (mostly because Roberta Williams didn't write some of this game. However King's Quest 5, is awful. Firstly: The voice acting, I know this was early but it is awful, the actors are clearly just Sierra employees. Secondly: Cruelty, Sierra games are known for being cruel, but this one takes it to a different level, to the point where it makes no sense how you solve a puzzle, or what you missed. Rant over, I still think these are worth buying, just don't get your hopes up for the 5th one.