MM spec is not viable. Not yet.
MM scaling is the best among all specs, and if things remain unchanged, in next content patch well see all top hunters changing to MM soon. Also, MM requires you to spend some time trying to get the best stat combo - and it is not this clear as in SV or BM. Both Mastery and Haste are pretty good, but Mastery doesn't need to go very high, and Haste has nice DPS plateaus you may consider reaching.
Anyway, I suggest to try this spec, even now. You will be below BM and SV (not far below tho), but you will graps the quite complicated concept and won't have trouble swapping specs soon. MM is always a winner, whatever happens.