I agree Airmiles are best used for gift cards. Keep in mind also that if using them for flights, you can't cancel and get your miles and fees/taxes back unless you purchase their cancellation insurance (which has restrictions as to eligible reasons).
The flights they will offer you are also the cheapest available. For example if there is a non-stop for $500 to your destination and an itinerary with a stop selling for $300, there system will only be able to offer you the $300 flight. Their availability is based on the cost of the tickets.
Lastly, because they are based on the lowest fares, they can only be used for coach, never business class.