I went from a 81 25 HP Evenrude, down to a 90 9.9 Evenrude, and saved at least $150 is gas last year.
I would think that 65 Merc will suck back the gas, and pollute like no tomorrow. As for reliability. It could be a great motor, or if could be a lemon. You won't know until you get a few hours on it.
Not worth is IMO. "edit" Unless it's DIRT cheap.
If you want a stronger engine, I would suggest something in the 90's. If you can't afford it right now, then start saving. Pick up something cheap at the end of this season??