I was playing ArmA 2 for years before this DayZ mod showed up.
My opinion of the mod: It blows, completely.
ArmA 2 has never been a very stable platform for multiplayer - modders have done a lot with it (just like they have here with freelancer), but it still has a lot of core issues that simply cant be fixed.
I love zombie games as much as the next guy, but after 20 minutes playing DayZ I gave up on it. First off you start with no weapon, and there's no melee combat included in the ArmA engine - meaning you cant even fight back with your fists if a zombie attacks you. Until you find a gun, you're completely helpless, and the 'stealth' system they tried to include in DayZ is flaky at best. Second are the issues with zombies 'wallhacking'. Because Arma was never really designed for fast paced, up close fighting (it's a military simulator, most combat takes place at ranges greater than 100 meters, and it's tactical rather than twitch based). The zombies are able to run clear through walls and buildings like they arent even there, while you (the player) are inhibited by those obstacles. Barring a door off and locking yourself in a concrete building for the night sounds pretty good in theory, but in the game the zombies will just run right through those walls and chew your face off.
Lastly, there's a huge issue with griefers and spawncampers on most of the popular DayZ servers. Players will work their way up to a sniper rifle or something similar just so they can perch up on a coastal mountain and take shots at brand new players logging in who have no way to defend themselves.
All in all, on a rating of 1 to 10, I give DayZ a 2, and only that high because of the concept the mod tries to build itself on. They should've chosen a better platform.
That said, you can find me on the 7cav domination and insurgency servers for ArmA 2 on a daily basis.