You might want to try out
http://www.snacky.fi , they're a chain but better than McDonalds, Hesburger etc., also The Chili's restaurant (not the multinational chain) in Espoo
http://eat.fi/espoo/chilis . I've heard praise about the burgers in Sports Academy, but haven't had one myself. Personally I am not impressed by Morrison's, more specifically the taste of their burgers, plain and middle-of-the road in my book.
Edit: Just having returned from having the best burger I've yet eaten in Helsinki, an addition to the earlier suggestions; Ravintolalaiva Wäiski
http://www.waiski.com/ The burgers are great and reasonably priced (for their size and quality) at 10-14 euros, including proper self-made fries. À la carte during evenings is open until mid August for the summer, but burgers should be available during lunch time as well (open all year round).