I have used the Merrick Farrow deck in the last two runs through the campaign and I can say without any hesitation that it is a incredibly fun deck to use. Having two cards that require 0 threat to use is really nice. Most groups will never take the Dark Pact offer, but the Dark Mark is one of the best cards in the game for bypassing defense. Plenty of hits will only do one or two damage, and transferring it turn after turn to the marked character keeps that hero in check by forcing him to constantly heal (or stay next to the healer).
Masques can also be a game breaking card. For a single threat you get to force a auto miss...yes please. How many missions come down a single roll for all the marbles? Its particularly good for negating powerful mage and warrior heroic feats in the second encounter of a quest.
Grasping grave can be really good as well, but I often find that the fortune tokens for the heroes are not worth the reduction in healing it gives. Still, if a hero gets off by himself, I could see its worth.
Bolt from the Blue is not worth it. If it was an auto hit then it would be a no-brainer, but since the heroes get the fortune token regardless of if you get the range or not, a single blue-yellow hit is not worth it to me.
Summon Merrick - No, don't.
Mystic Might, Cabal, and Thaumaturgy - Meh, you have to stack your OL deck with "magic" cards to really take advantage of these, and that is not a deck I am big on running. Although I am currently running an "infector" deck with Basic I, so maybe it will change my mind on these.
Overall I highly recommend this deck. It tends to be a very low threat requiring deck and gives the OL some nice abilities without feeling cheap like Baron Zach's deck.