The Good: Free wi-fi, fairly easy to get around (Terminal 4), Decent dining options (Terminals 3 and 4)
The Bad: Parking in their garage can cost you dearly, relocation of offsite parking shuttles, terminal 3's somewhat confusing layout, Terminal 2
Being a person who frequents this airport, I can tell you it is not Incheon nor Dubai. However, it is a decent sized facility that can accommodate larger aircraft. Many of the gates now have newer jetways to accommodate the larger A321s and could easily accommodate the A330s/340s.
Food options are decent, even though scarce in some terminals. However, that is changing. They are building restaurants such as McDonald's and adding new ones such as Starbucks. Even baggage claim has a Starbucks.
The above is mostly about Terminal 4 but Terminal 3 has good food options but the terminal is very dated. Has a fairly confusing layout to the unfamiliar.
Terminal 2 is even more dated. Shame that United and other carriers use this terminal when spaces are available at 3.
One of the biggest problems I have with the airport is how the off-site parking shuttles are positioned. The best way to describe the area is chaotically dangerous. It is literally at the end of the airport where traffic at different points merges and in front of the corporate cars. Just imagine a parking or hotel shuttle trying to get in and a corporate car trying to get out with traffic whizzing past between 35-50 mph. Herding cats would be more orderly.