Cool fellow but tricky to use though.
Ideally you play him when you lots of time left before a timer advance OR when it's right about to advance. Remember that if doom appears in the same round that the timer advances anyway, that doom goes away too without issue.
This means that you can trigger David in the round's the agenda advances as a "freebie". If you can do this just once you've just spent 1 resource to get a +1 Willpower proc' for a round, 2 health and 1 sanity. Every extra time you can pull this off is pure grade-A, gravy.
The trick is to know when to kill David off, despite his powerful ability you must keep in mind that like other cheap Allies (Art Student, Dr. William.) he is first and foremost a throwaway character. If you manage to squeeze 2-3 resources, one or two uses of the +Willpower and then soak a 1/1 attack with him you've gotten tonne's bang out of this one card!
The big risk in using David is falling into the trap of stacking doom on him frivolously, the way to do this safely is to, as already mentioned, make sure he dies a timely death. In a scenario where you've got 4-5 rounds between advances you can stack 2-3 Doom on him for a gain of 6 credits, in this case you've got to get in some deliberate trouble so that he gets punched before the big payday turns into a premature nightmare.
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Pros:
-Consistent benefit when used safely.
-High yield benefit in the right circumstance.
-Moderate benefit in the wrong circumstance.
-At worst a 2-point damage soak or 1-point horror soak.
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Cons:
-Game-loosing if used wrong.
-Dead-draw in the lategame of each scenario unless you can find a use for raw hitpoints.
-Inherent risk, even the safest bets can fail every now and then.