For starters, remember that the first person needs to pay the current full price of the ticket. You can then take 2 people as companions at $99 each + taxes/fees.

This only applies to economy fares. It cannot be used for business or first class.

There have been reports on here that 500-mile upgrades can be processed for elites. However, I haven't seen any reports of upgrades being allowed if you tried to upgrading using miles.

As for good uses, for me, it is for trips where the rates usually run pretty high. So, for example, out of PHL, I would never use it to do PHL-MCO. Those tickets are usually around $200 round trip and it would be a waste to use this for it. However, there are some places, especially out west, that usually run at about $600-$700/ticket. One place I'm considering for this year is Jackson, WY, because it is usually an expensive ticket, so the certificate saves me a bunch of money.