My 18-month old has made 4 round-trip flights so far and all have been with the infant half-price fare. The vast majority, if not all, of the major airlines offer this fare - you just tell them you want an under-two fare and it will be half of the price before taxes/airport fees. So far, none of her tickets has been over $135 which seems like quite a bargain to me.
I would never consider flying with an infant unless I had a seat for her. It is hard enough traveling with a little one without also worrying about keeping them content on your lap and safe as well. My daughter is used to her carseat and doesn't expect to get out of it so it makes the flights soooo much easier on everyone.
I do agree that the airlines should do a better job of informing passengers about the infant fares but I guess they are never in a hurry to turn away revenue. I certainly do think they should always tell you about the fare when you mention flying with an infant whether you request it or not.
I guess I don't understand why people complain about buying an infant fare but still manage to pay for a full-fare ticket when the child turns 2. The infant fare really doesn't significantly add to the cost of the airlines tickets and when you consider that the child is generally free for everything else (getting into WDW, eating at buffets, staying in your room, etc.) it makes it an obvious decision for me.