Aaron86: Is Arma II seriously only worth it for the DayZ mod?
Depends. Can you put up with a lot (and I mean a lot) of people that like to mess with newbies and kill them? Some shoot other players on sight.
Aaron86: So if I wanted an ultra-realistic military shooter I should still pass Arma II? Would Arma I be just as good if not better?
(And of course I'm mainly interested in single-player.)
If you want an all-around shooter, it's probably the best realistic one you'll get right now.
However, for its whole, there are better parts.
DCS Black Shark is a more realistic helicopter sim by a long shot.
DCS A10-C is a more realistic military plane sim.
Steel Beasts Pro PE is a better tank sim, and there are also alternatives like
Steel Armor and
Steel Fury which are realistic tank sims with LOTS of
mods, and apparently are some of the best tank sims ever made (and are single-player only, so that'd be a plus for you).
If you want a tactical simulation, they're ugly as sin, but
Tigers Unleashed and it's sister product
Point of Attack 2 are apparently the most realistically modeled tactical simulations ever. There are also even less ugly simulations like
Achtung Panzer and
Theatre of War.
EDIT: Oh, totally forgot
Falcon 4.0 and
FreeFalcon, which is based on Falcon 4.0, but free, with improvements. Apparently they're fantastic modern combat flight sims. Haven't tried either myself.
But yes, if you want everything in one, or a realistic infantry shooter, ArmA II is probably the way to go right now.