Okay, it's an attractionpark and it's very close to the city. But that's about it. It's mind of rundown, no atmosphere andn no special attractions. Though not cheap.
If you want to go to an attractionpark, better go to some other place (Germany, France, the Netherlands).
If you want to do sightseeing in Helsinki, better spend your time enjoying real sights.
If you have small kids, it might be worthwhile going there for an afternoon.
I think that this tourist trap is worth of visiting. I loved this plave when I was a kid and it was absolutely the highlight of summer to go to Linnanmäki.
This park has it's own colourful history and there is a wonderful wooden roller coaster, which is not that wild as it's cousins are in U.S. but it's made of wood and you can really feel it.