Although living in Helsinki, I had never visited this place before, and decided to do this with our 3-year old on New Years day when almost everything else is closed. The building looks very big from outside (expected something more of the scale one usually gets in Finland ;-) ), but inside there is only one floor, with 4 separate medium-sized exhibition areas and the planetarium/theatre. Two of the areas appeared to be permanent: mostly interactive physical "experiments" and computer-based exhibits. The science part was not very strong in most of these, i.e. you would not really learn anything new, although many of the experiments demonstrated some basic physical principle or a visual illusion. Our 3-year old was interested only in the following: the ship sinking when bubbles are produced in the water container, the two rockets that are shot respectively with manually produced hydrogen and pressurized air, the car-driving simulator, the foam-cushion hut and cubes, and from the "mental health" exhibition the room where you click on a pair of eyes in the darkness to reveal a forest animal. Also, our 3-year old was happy to have his face printed on a coin, of course, and was happy to see the rats play basketball.
As pointed out in another recent review, the Mars exhibit was pretty boring for both adults and small children. Possibly there would have been something moderately interesting for 10-year olds who can read the explanations themselves and for whom many things are new and exciting. Nothing in the exhibition was really about Mars. It was mostly some basic information about issues related to space travel. The mental health exhibition was still less interesting. I think the low funding level and visitor numbers explain this.
The positive thing was the quality of the interactive exhibits: these worked well and had explanations in six languages (Finnish, Swedish, English, Russian, and two others that I can't now remember.)
Cannot comment on the planetarium, as we did not visit it.
Overall, quite expensive considering what you will see. If money is not an issue, you could spend here an hour (or with curious 10-year old maybe two), but don't expect too much of science.
If you own or manage Heureka, the Finnish Science Centre, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.