I've only been out on my own, been invited by the Koreans from my bike shop a number of times, but I really don't enjoy the large numbers they tend to gather in.
I've done Baekyang mtn. a couple times, and Mt. Hwangryung. Pretty easy trails overall, well maintained jeep trails, a little too much crushed stone makes it tricky on the way down, but Baekyang goes up to 600m which is nice~
PM me if you're interested in coming out this way, I have Tues. and Thurs. afternoons~evenings off this week. i also know of some cool spots outside of town, could cram 2 bikes in my jeep I'd say. . .