OK. I read the headband's description as resetting a character' INT to 17 (or 19, I've seen another such item mentioned).
My plans to play a STR16 DEX14 CON16 INT8 WIS12 CHR10 Human Wizard are thwarted! :P
Your understanding is right. The headband raises your Int. to 17, giving you a +3 overall. It's not less helpful if you started with an 8.
Although, a 17 isn't great for a wizard in the long run. It's certainly okay for the early levels, but later on I imagine that 17 will feel awfully low when you're wanting a 20 instead, and there's no way to upgrade.
It seems to me that the headband is pretty useful for an eldritch knight or an arcane trickster, though. It's a solid stat for a class that's multi-stat dependent. That said, I just haven't been that impressed with those subclasses in comparison to their alternatives. The Thief is so much better than the Arcane Trickster, in my opinion. Same with the Battlemaster and the Eldritch Knight.
Basically, I'd argue that the headband isn't OP, not at all. In fact, as a player, I would much prefer a headband that gave a +2 to Int. (Heck, even a +1 would be better to my way of thinking.)
