Well that was a nice thread! Membrillo! As a (half) Spaniard I can tell you, it is very common chez nous. And may I tell you a secret? If you do it yourself (which I did, and it is nice, though it does indeed take longer than the recipes say until you reach the setting point, and it does not boil softly but blurps and makes the whole kitchen sticky and it burns badly on your forearms) it is the ideal excipient for adding just the right amount of haschisch. Cut it into tiny squares, dunk it in dark chocolate, put an almond, hazelnut or pine nut on top if you fancy before the chocolate settles, and it not only lasts forever (that is: until you eat it), but it makes you really happy. Do you read German? I guess so, judgind by your nick, so this is the recipe for membrillo I recommend. You make jelly out of the coocking liquid and membrillo (Quittenbrot in the link) from the solid parts. One recipe, two products. I. love. it.
BTW: The lemmon in those recipes is just so the end result looks better and does not run dark. It is like with artichokes. It should not change the taste, or you have used too much. A spritz is enough.