I visited Helsinki in early June and had great weather - t-shirt and a light jumper for walking around. Obviously there's plenty of daylight. Everyone was out socialising, the old Olympic pool (outdoor) was great. Lots of rooftop bars with great views and usually a list of which hours the deck will be in sunshine. But I'd love to go back in winter and go north to see the borealis. Depends what you like.
Whenever you go, check out our travel safety guide here: http://safety.worldnomads.com/finland
Phil at the safety hub over 4 years ago