I replaced a rear D606 with this tire. I found the off-road performance as good as the D606 in dry sand, hard pack and rocks. Not as good in mud.
On the street, the MT21 is far superior to the D606, which I found to be a dangerous tire, especially for cornering. The D606 will slide out suddenly with no warning with very little lean angle, but the MT21 lets you lean further, and loses grip only gradually, so you have a warning and a chance to correct.
The D606 wears slightly better than the MT21, but I'm up to 5,000 miles on the MT21.