I've focused by eye most of my printing life and only found the grain focusers to be helpful when I went to 4x5. But I don't need glasses, my safelights are red (and therefore slightly dim), and I open the lens all the way up to focus. I've also spent a great deal of time looking through microscopes and am used to focusing through something to find the focus.
I agree that it's worth getting a really good one if you do it. Check the prints you've made to see if you can see the grain. If you can, then you're focusing the enlarger just fine.