I've hardly played much of the singleplayer at the moment, just got used to the controls on it and then cut and run to the multi, but I didn't find it that bad. Like I said, I hardly have any experience of the Halo games so I found it nice and refreshing, a lot more so than most FPSs. I like the way the levels are set out in such a way so that it feels like you're at fault for dieing because you were careless (like, you kill a few baddies, spot a sniper and kill him and then get cocky and run out only to be popped by another sniper) so I'm liking it. I also like killing the brutes
their grenade launchers only cause real problems if you aren't careful, most of the time a measured couple of snipes to weaken them and then a rush with SMGs is good enough to take them down, but they have a nasty habit of running behind obstacles and then second-guessing you when you try to oust them. I like it! This is only the jungle level as well, I can't wait for the bigger, open-plan vehicle maps (the big ice level with the tank in Halo 2 springs to mind).
Also don't see your problems with the graphix, they look pretty nice on my telly. A little rough round the edges, but it never caused me any probs at all and I'm happy sacrificing a little for nice, smooth gameplay.
More to the point, what are you doing faffing around on singleplayer? If you'd come on and plough through it on co-op you'd probably enjoy it a helluva lot more (I've been saving myself the co-op until you come on, anyhow)!