I've had 2 pairs of Metalheads fail on me in the past year... The entire sole (not just the inter-changeable bottom) started to just separate from the rest of the boot. Each time, it started on the inside of my foot and worked it's way around to the front and outside.
Korkers will replace them for free (I go through my local shop) however this time I'm going to opt for the Chromes and pay the extra $$$$. The Korkers rep said that's the best solution to the wear and tear problems with the Metalheads. I know a few ppl (including some guys that work in the shop) that have had the same issues with the Metalheads, gone to the Chromes, and been ok.
One other issue, while crossing some pretty gnarly water, the removable sole part (felt + studs) actually started peeling back. They replaced the soles for free, but I have read on some other forums that can be an issue and to just drop some of your own studs in to help secure the rest...
durability issues aside, great boots. Love the dial system, as some one said recently pull all the slack up to the top before you tighten for the final time. Comfy (I wear a size 14 boot, 13 reg shoe). Felt plus studs give some great traction, but the regular rubber soles are ... non existent. Wore them one day on the West Branch, felt like I was sliding in ice, couldn't confidently wade anywhere.
I'm usually in the water ~100 days a year. No, not for 8 hours. Lots of times in the Summer it's just for a couple hours after work. I'm not an overly aggressive wader, but I don't stay tied to the bank either...