This was a good game. Probably one of the best "bad games" I've ever played.
My expectations were very low, but this game pleasantly surprised me. Killzone is the Netherlands' greatest cultural export since the Dutch Reformed church, Dutch apple pie, and them being confused with Germans (as in the case with Dutch apple pie). However, that mostly applies to Killzone 2, 3, and maybe the PSP and Vita games. Killzone 1 is the red-headed stepchild of the series, because it's more like a very rough draft of the series. The clunky and slow gameplay that fans had come to know and love is still there, but it hadn't yet been refined.
First thing's first, this game is nothing like Halo. If you go into it expecting it to play like Halo, you're going to get absolutely pulverized. The name of the game is to take it slow and steady. Weapons take forever to reload, and switching weapons violates Gaz's golden rule (remember: switching to your pistol is always faster than reloading) because switching them triggers a gun-cocking animation that takes a fairly long time to get through unless you cancel it out by pressing L3. The enemies are also incredibly spongy, but their AI is completely brain-dead so you won't have too bad of a time if you learn their movement patterns and slowly plow through the levels.
The thing that Killzone 1 does differently from its successors is the character select system. Throughout the game, you unlock four different playable characters. There's Templar, who is the basic soldier; Rico, who is the heavy gunner and acts like Wakka from Final Fantasy X if he were pasted into a military shooter; Hakha, who is similar to Templar but has a knife, a different loadout, and can pass through Helghast mines and explore certain areas; and Luger, who is a woman. Like all women, she is sneaky and has infrared vision. Each of these characters has their strengths and weaknesses, but I would recommend sticking with Rico if you don't want to pull your hair out. He has the best starting weapon in the game, a chain gun that melts entire hordes of enemies and doesn't need to be reloaded (it only has an overheating system but is easy to control with short bursts), and he can take a beating. A sneaky grenade can completely annihilate Luger in one shot, but Rico can take grenades like they're nerf darts. His only drawback is that he's slower, but this is not a run and gun shooter so it's not that big of a deal.
The weapons feel... off. The bullet spread is off the charts, leading to the assault rifles not really being as viable in mid-long range encounters. The sniper rifle is the only really reliable long-range weapon, but that one is much less mobile and much rarer. This game would have felt better with either less bullet spread so you could more reliably get headshots and be rewarded for accuracy, or a mid-long range semi-automatic rifle that's more accurate than the fully automatic rifles. The shotgun also kind of sucks, and is borderline useless in the late game against tankier enemies.
This game also has a degenerate checkpoint system, like most sixth gen games. For some reason, the last few levels hand out checkpoints like candy, but other than that, the rest of the game will force you to restart 10-15 minute sections if you die.
Overall, I think this entry in the series is overly hated. I'd give it a shot, but don't expect something on par with the second or third game. I played the HD version on the ps3, which is the best option if you have a ps3 or the power to emulate it.