We've used Finnair several times to fly to the Far East and always thought they were fine. No problems at all, and better than many other major carriers we've used. Helsinki is a nice airport if you need to change planes. The shorter flying times is apparently due to the curvature of the Earth - fewer miles to cover by flying in an arc (via Helsinki) rather than directly east... I wouldn't dream of paying an extra £600 NOT to fly Finnair!
We are going to Thailand next July for 2 and half weeks and need to sort the flights. Finnair are coming up the cheapest ( we will be departing from Manchester) and the shortest flying time, but have never flown with them and their reviews online are'nt the best , but on the other hand would rather not spend a further 600 or more just to fly with someone else. What would you do??