Usually it is, especially if you can buy when they're doing one of the deals. I seem to get emails about those at least monthly.

As noted, things to consider:
* You don't earn miles when you burn miles. So it could be more difficult to earn status.
* If you don't have status, you could be one of the first downgraded (or offloaded) in the event of equipment change, air marshal, irregular operations, broken seat, etc. They typically give priority to paid, status, moving down the line.

I managed to use miles to book QF biz SFO-MEL and QF F SYD-DFW for later this year. A paid roundtrip biz USA-Australia is about $5-6k; F is substantially higher. So I think I made out nicely.