An extremely fun and exciting addition to the game.
This card is kind of similar to Vicious Blow, in that it allows one of your attacks to deal +1 damage, provided that the attack hits. A very strong effect.
To flesh out the comparison a bit more:
--Marksmanship only works on Firearms or Ranged weapons;
--Marksmanship doesn't grant +1 to hit;
--Marksmanship can't grant bonus damage against enemies engaged to you.
--Marksmanship can target enemies one space away, which is very nice and can often save you an action;
--Marksmanship ignores Aloof and Retaliate. In this case I don't think avoiding Retaliate is that significant. You probably wanted to heavily boost your Marksmanship attack anyway, such that your odds of hitting were very high; this means Retaliate is not a huge concern. (And of course some targets won't have this keyword anyway.) Avoiding Aloof is a nice action-saver when it comes up. But this is going to be relatively uncommon because not many enemies both have this keyword and are reasonable targets for Marksmanship. (It's a bit of a waste to snipe a Whippoorwill).
On the whole, I think Vicious Blow is the stronger card; its advantages will basically always be helpful. Marksmanship's advantages are much more situational, requiring specific enemy keywords and specific enemy placement, which you may or may not encounter in any given scenario.
With that said, Marksmanship is still a very welcome addition to the Guardian class and something I will gladly run in addition to Vicious Blow. +1 damage is extremely powerful. (This will be obvious to any veteran of Arkham Horror LCG, but if you want an explanation of why +1 damage is so good, read my review of Vicious Blow.) So Marksmanship will be well worth running, provided that you have a lot of Firearms in your deck. Ideally, these will be really strong firearms like Lightning Gun, because that increases the chance you can get maximum value from Marksmanship by sniping a powerful enemy one location away in only one shot. (The generous to-hit bonus on something like Lightning Gun or Shotgun will also reduce the chance that you miss and waste the card; this is particularly welcome since Marksmanship itself does not help your attack hit.)