In general, I would rate it as excellent, with a few caveats:
1. English (or other foreign language) resources are not always easy to come by. This is because the local diving community is really oriented towards Korean divers (as it should be, I think).
2. Rental gear is not as plentiful compared to how many dive shops and outfits there are. This is because many divers own their own stuff. This is not to say that you can't rent equipment. Rather, sometimes it may be hard to find your size, or the stuff that a shop has may be a bit worn (not to say that it is not serviced properly or taken care of...I just mean that it is a bit tired looking sometimes).
3. One aspect of Korean diving that may be a bit "behind the times" is their attitude towards preservation or sustainability. Many people dive with the intention of catching and eating things that they find. I don't think this is necessarily wrong, as Korea has a long tradition of fishing in this way (see the video above), but I think sometimes this attitude comes at the expense of the environment. I don't have a lot of experience or anecdotes about this, and there may be lots of divers working towards sustainable outcomes that I am unaware of, but it just seems to me that there are fewer efforts to consider the environment when diving than I have experienced in other locales.
4. The oceans around Korea are not the easiest waters to dive in. Visibility can be limited, currents can be strong and change quickly, and water is generally cool or cold. When combined with limited language abilities in English (or our limited ability in Korean), dive instructors are hesitant about taking divers out when they are unsure of their abilities in Korean waters. I actually think that this is a plus, as they are erring on the side of safety, but it can make finding an outfit willing to dive with you a bit tough initially.
These are just some of my opinions, and others may contradict my ideas or have other experiences to share. I hope this has been helpful.