Do you mean the ones that HD typically has outside for less than $4? They're about 4"x6"x8' long and pressure treated- right? I've used those all over my property since we moved in 5yrs ago and havent had one rot out yet.
ive used them all over the farm and they work great, yeah you might have a few twist or bow but ive had treated 4x4's twist and warp also. In my run i have Cedar corner post that i cut off my property but i use landscape timbers as brace posts for my cedar post. If you have access to some cedar trees cut your own posts. Cedar make awesome, long lasting posts.