Main point - cannot stress enough - do NOT transfer until you've found the flights you want. THEN transfer to pay. You cannot reverse a transfer.

Don't discount partner arrangement as well - for example, you can book *A first class via Continental - also VS and some others; OW via British Air or Mexican Air (IIRC); Skyteam via Delta, for example. Basically, each transfer partner has some level of access to their partner flights.

You'll usually get far better value with the transfer vs. pay with points.