Welcome to Librarium Online!
I've had an old one eye for a while, and have just been playing him as a normal carnifex. Is it worth it to now use him as actually Old One Eye, or is he not any good?
He may be decent. Very expensive for what he does, but that's true for all nid MCs now.
I was lucky enough to get old one eye a few years ago when he got written out of the previous codex. Got it for under 20 dollars on clearance! Granted he's still sitting on the shelf untouched, but still...
OOE didn't strike me as a very strong choice. I'd rather just get a Trygon Prime or another mawlock if i wanted a good MC. Though you never know! he may be quite the beast it's probably best (and most fun) to try him out.
"I have seen you humans, trying to forge an Empire in the name of a corpse"