The MBR trail is one of the three advanced trails at the Coed Y Brenin mountain bike centre. For most of the way it follows one of the other advanced trails called "Beast" which is longer. So MBR is really just a shorter version of "Beast".
About half of the trail is forestry roads. According to the promoters the trail is a "predominantly open and flowing trail" with "long sweeping descents", but that is more or less exactly what it is not! There is only about 200 metres of fast flowing singletrack that has berms and jumps. The outstanding feature of the purpose built part of the trail is the cobbled rock slab sections with lots of drops when they go down hill.
With all those rocks and the forestry roads, the trail is all-weather and at 18km makes for a good workout. If you want more tackle the Beast which covers much the same ground and adds another 14km of often challenging trail.