Wow, interesting. How long did it take yours to start coming apart? Do you do lots of suwari waza?
We normally do 2 laps of (anywhere between 4x6 tatamis to 6x6 tatamis) shikko, and perhaps average one or two suwari waza techniques, and every class ends with (what we call) kokyu ho.
The Bu-Jin (12oz?) pants began failing after about 2 years, which included a 3-week 2 sessions per day every day trip. They have outlasted every other pants hands down (so far), but I will no longer buy any pant with a knee patch area that does not go all the way to the hem.
It was by pure luck the Iwata ones did, but once I tried one lap of the mats, putting on any others pants is not something I look forward to. Maybe it's just my sharp bony knees....