Superb
Well.... where to start... First of all, excellent work. Was truly fun from start to finish, and the game actually did get my pulse racing at times from trying to kill off the zombies as quickly as possible. Plus just seeing a wall of them approaching your humble wall is pretty intimidating.
Controls were very smooth. Aiming was a breeze, though it seemed to get a headshot, you had to land the crosshair on the neck, rather than the head. It might have just been the angles I was shooting at, however, so we'll leave that one up in the air. And besides, it wasn't a big deal (unless you just love seeing massive blood sprays like I do XD).
I appreciated the RPG-ish feel to the game, it helped make the gameplay more intense than it already was. I admit, I was unnerved the first time few times I played, trying desperately to keep those zombies away from my poor wall. And it also did unnerve me whenever I lost a survivor during the day hours while searching. The extra firepower they add seems to be quite significant, particularly the sniper (yeah, guess who's also the first to go when you have 7? XD)
I found all the weapons useful in their own way, and I enjoyed that. The ability to just pick and choose which weapon I wanted based on preference helped make the game more enticing. Even the starting pistol had a surprising amount of longevity, despite being significantly weaker than the others. I've actually managed to play the game the whole way through using nothing but the pistol, though I did have to repair obsessively.
Which brings me to my last point. The replay value on the game is astounding. Even though it's the same game whether you play it for the first or twentieth time, it's still great fun to play. Devising little challenges helps make the game even more fun. The "no survivors" challenge added quite a bit of pressure on me, I didn't appreciate how useful they were until I didn't have them anymore. ;(
...Wow, that was long... Anyways, long story short. Very good job, hard to believe this is your first flash game. Keep up the good work!