It all depends on how they are used.
An example of a good retcon is during the extremis saga of an Iron Man comic, now we know Iron Man came about during the Vietnam war, but to sort of keep things a bit more modern, Iron Man's origin came from the Afghan war during the early 90's.
It doesn't contradict anything, and if you know the comics before hand, it doesn't take away your enjoyment from them. It just helps keep the comics updated. The stuff before hand is all still canon, it just now takes place a bit later than we first expected. (It does get weird when you get to stories that follow on from Tony's origin through). But it's more of a passing thing, it doesn't take up a lot of the story and is only there so newer fans don't have to read 50+ years worth of comics.
As we've seen before, Jet in Fallout 4 makes no sense at all. It ties in with a character's story from the second game. Again, I know Tony Starks origin also ties in with his character, but it's much easier to pick up a series of games of which five are considered canon, than a series of comics which have been coming out on a regular occasion since the early 60's. Plus, Jet in Fallout 4 doesn't do anything to update the canon, it's just there as an inconsistency. The worst retcon in Fallout 4 has to be that Ghouls don't need water to survive, as this was a major plot point in the first game.
It's not an update but contradicts Ghoul biology that's been established for years.
But then we get a series like Metal Gear, which are full of retcons. But the difference is that these were retcons in that the previous ideas are still canon. An example is Liquid Ocelot, which was retconed in MGS4 as being Ocelot on drugs. It doesn't take the enjoyment away from MGS2, in fact, it makes a lot of sense (and playing MGSV makes even more sense of it). But it doesn't contradict anything that's already been established.
Anyway, that's my opinion on Retcons.