Heitz - there are definitely still some fair to good trails to ride in the Mercer area. Even with the heavy traffic and warmer temps, my group found some very nice trails on Saturday afternoon. You just have to avoid the high-traffic trails, like 8 and 17. For example, the two trails to and from the Flowage were very good to excellent -even on Saturday afternoon. (The Flowage, however, was tricky - don't pin the throttle. There are smooth flat spots on the Flowage, but there are many very rough spots and they just appear out of nowhere.) Also, the Iron County trails North of Mercer and West of Pine Lake held up very well.
Its not the best riding Mercer has seen this year, but defintely well worth the trip. Also, you could always trailer 45 minutes North to the UP if somehow things go to garbage.