I grew up in the Northern Kentucky area, and i have sooo many fond memories of visiting Kings Island. It is a GREAT park to go to if you like thrill rides, ESPECIALLY roller coasters.
You have the classic BEAST roller coaster... a wooden giant... and though I'm sure it's probably been beat by other woodies in height and speed and all that - this is just a classic, and you can't beat it for fun value!
There are other great coasters in the park, too... Face/Off, the King Cobra, Vortex, Outer Limits, Son of Beast (the world's first looping woodie!), the Racer... and I'm quite sure I'm likely forgetting some...
In addition to thrill rides, there are also your typical park rides that are fun for the whole family. For kids they have a whole area all their own, based on Nickelodeon characters and themes.
There's plenty of places to eat - including yummy Cincinnati La Rosa's Pizza!!!
It's been awhile since I've been... in the past few years, it seemed like the ticket prices kept going up and up - but this year, it looks like they've actually gone *down*!
My only complaint about the park would be the crowds. I can remember waiting 2 or 3 hours in line to ride a roller coaster... 45 minutes or an hour for something like the Octopus or the swings... This can really detract from the fun you're able to fit into your day. I'd definitely advise going on a weekday if you go... I would expect the crowds to be less then.