I was quite astonished when reading about their insane growth. Most of the companies we talk to in The Netherlands (Telco's, E-commerce, Banking / insurance, travel) are using VWO, Sitespect and last week I heard two clients talk about Maxymiser. Not one time I heard them talk about Optimizely. Seems there is still a lot of room for growth (at least in NL).
We (usabilla.com) are using Optimizely with great satisfaction though!