Rarely worthwhile. As the previous posters stated, basically you have to buy a ticket that would probably cost just as much, if not more than a discount Business Class seat on the same route, *and* you have to have scarce Aeroplan (or Star Alliance-member carrier) reward space available on the flight. An additional twist is that for Aeroplan to process it, such inventory has to be exposed to Aeroplan, which sometimes doesn't happen as carriers selectively expose inventory to each other as a means of reducing redemption imbalances. If you actually can make it work, I'd also recommend buying lottery tickets.

Aeroplan is undergoing some major revisions supposedly in the next year or two that may make using points for upgrades more feasible, but until those are announced, all you can read here is lots of speculation.