I don't have the cards with me, so I'm recusing myself from the conversation for the time being…

What cards offer the OL the best "bang for the buck" from an XP spending perspective? It would seem, to me, that the saboteur path ranks very highly… Mostly because the majority of the quests depend very highly on mobility. Since saboteur offers numerous methods to slow down the heroes, that is a terrific benefit.

Has anyone chosen cards and found them to be more useless than anticipated?

More useless than anticipated? Yes. The Magus cards. The level 2 cards are okay, but I usually leave out the level 1 cards when building my decks.

Though I do really like the Warlord cards.

I agree. Warlord and Sabateur are the ones that interest me the most. I've concluded that my choice of "open group" monsters is more important than my choice of cards when it comes to giving me an edge as the OL but getting the right card at the right time is always good.

Magus is a mixed bag but the card drawing cards are amazing. Lvl e magus searching out a card is amazing, the damage is just gravy on the meat. I have won a game off the level 2 card as well.