I use useful more than anything besides like I think.
For me it means the post has utility or is of use.
Or what it says "useful" but using useful in a sentence almost comes across as condescending because it would imply other things are not useful or have no utility.
Closest thing to helpful I suppose or moving the thread along/adding something. Typically, it stands out as quite useful rather than just marginally. It could be useful for me personally or in general...even when I don't like it, it still has utility. Something like a difficult sermon to hear or a hard truth for instance. Useful, but not necessarily appreciated or "likable".
It is usual for me to find at least one thing useful in most things, so that's personally what I look for. Something edifying is "useful" too. It'd be nice if it wasn't a wrench or maybe a bit more colorful but it vaguely says something and therefore is "useful" to me instead of quoting sometimes