Stuart B.
Yelp
This is definitely Modern Art- don't come here expecting any portraits in frames or old masters.
Even the building itself is bizarrely-shaped, both outside and in- very spacious, very very white, and with extremely odd acoustics that mean you can hear people talking several rooms away.
It is split into lots of differently-sized exhibition spaces, many of which are rearranged on a regular basis, so you could check in advance to see what you're going to see.
There's a lot of emphasis on video & sound in the art, and several rooms full of video art, as well as multimedia experiences that are a mixture of text, sound and projections.
Each exhibition is introduced in reasonable detail, in Finnish and English, and a lot of it is rather fun.
There's also a very good, if expensive, shop, which will sell you a wide range of art books of course but which is also worth visiting for its novelties- postcards, badges, and little gifts, all art-related but with a definite sense of humour.
If you're in Helsinki it's definitely worth a look.