I always say that it is worth having at least one of your displays professionally ISF calibrated in your life. I simply think that it is worth everyone's while to see a truly accurate image from at least one display. That way, you get used to what your images are
supposed to look like. After that, you can always use that calibrated display as a reference for future/other displays.
Without a colorimeter, you can never really perfectly dial in the colour points. It simply cannot be done by eye. But the flip side of that is that if your colours are just slightly inaccurate, you'll never notice just by looking at them!
So it's worth getting used to accuracy by having one ISF calibrated display in your life. But, after that, you can pretty much adjust future/other displays on your own. They won't be perfect, but you can get them to where they look the same
to your eyes. For most folks, that's perfectly satisfactory and I'm totally fine with it myself.
But if you've never seen an ISF calibrated display, I definitely think it's worth the money. Most people's
idea of a "good looking" image is pretty far from accurate. So do get used to an accurate picture by paying an ISF tech. But once you're used to it, I personally don't think you really have to worry about it if simply enjoying your images is all you're after