I've only once had an issue with K2's customer service over anything financial (an autoloot voucher which got deleted in a bug way before it's expiry time - I did eventually get a KC refund, but it took like 2 weeks). They're not bad on tickets to do with payments and PUS items.
The standard of their CS on things like bug reports, cheat reports, in-game help, is abysmal though. The biggest problem is that their CS staff are mostly sat in an office in India and have probably never even launched the game. If they had in-house support staff it'd probably be better. Which is why the few talkative members of staff we know about (Linus, CaliMaestro, Peregrine) tend to be able to get more done.
But K2 are a small company, where their margins matter - they know they don't have a new and amazing product, so they have to keep investment at a minimum, as they're not likely to get a huge influx of new players any time soon. The problem is, they've kept the investment too low for the last few years. If they'd spent a bit more in the earlier days and kept the game more bug free, then they'd be in a better situation now, but that's history. Is you plan to start an online game, you can use this as a case study in how to make the best of a bad situation.