To respond to you, I've rooted phones before, but never the HTC One. I didn't root the one HTC phone I ever owned, the ThunderBolt, because it was too complicated. But especially this long after the launch of such a popular phone, there are guides all over the web that make it really simple, including some do-it-all kits, where you plug it in, turn on USB debugging, press a button, and viola!

As for Custom ROMs, you can root without custom ROMs. You can't install a Custom ROM without rooting. If I were you, I'd first focus on rooting and making sure it's working properly, and then determine a Custom ROM. There are tons of great ones to choose from.

I rooted with TWRP and I like it a lot.