If you
absolutely, positively want some mag wheels on your MTB, then these are the only ones I can recommend:
http://www.aerospoke.com/order.php?cat_id=003
...the website says they're all out of stock because they're backordered due to the US military using up most of the available carbon fiber lately--but if you give them your CC#, they'll get you your wheels. Aerospoke has been around many years now.
Aerospoke mags have been used for MTB pro-level racing and do hold up to it.
However, they have a few disadvantages:
1) cost..... ~$720 a set, if you wanted disk-brake compatibles. You can get some
very strong wire-spoke wheels for ~$500 a set.
2) weight.... they weigh more than a good set of wire-spoke wheels will. And-
3) repairability.... if you hit something and ruin the rim,
the whole wheel is gone. There is
no way to put a new rim onto the carbon fiber wheel (-the axle may still be good, but that's not the expensive part--the carbon fiber part is).
~
