It's a great learning tool for when you and your friends are still getting the hang of the game but it's not popular with anyone who likes fixed points values and standard matched play.
In the 8th edition rules Open Way play comes with three very basic missions; Annihilation, Hold At All Costs and Death or Glory. I've played Annihilation three times as part of a small points escalation league but I have no experience of the other two.
Annihilation is a simple kill mission where you are trying to destroy your opponent's army within a fixed five turns. Simple but very boring. It makes a point of saying that both armies should be about equal on Power Level to balance it out though.
Hold At All Costs is also a kill mission but one where the player who has the fewest Power Level (the opponent should have about a third to double this Power Level) sits in the middle of the table and has to survive through five fixed turns.
Lastly, Death or Glory, is basically the same as Annihilation but the attacking player needs twice as much at least Power Level than the defender.
There is nothing to these missions really which is rather disappointing. After all, there is nothing stopping you from playing any mission with Open Play armies. I suppose that they are good choices for Open Play as you can't guarantee what each player will bring but I dislike the notion of higher or lower Power Level as some sort of mission mechanic or balancing option. I mean, you can play Matched Play style games based on Power Level anyway so why include these rather than just having a brief summery about Open Play in the rulebook?