Pros
I should caveat this review by saying that I was a US based resource and experiences can be very different in other offices. Also, there is a ton of variance between good projects and bad projects so two individuals in the same office with the same title can have a very different experience.
I was fortunate to land some good projects with some strong teams. I received great exposure to predictive analytics / machine learning and consulting while at Opera. I worked on some very challenging and interesting projects. Predictive analytics / machine learning / Big Data is a very hot space right now so experience in these areas allows for a lot of flexibility.
I also met some great people (also see cons). Work-life balance was very reasonable for me.
The worst is probably behind the company as the company hit a bottom 6 months to a year back and Opera seems to be building some positive momentum. We'll see if that lasts.
Kontras
A few years back this company was attracting top talent both across the globe. That's not the case anymore, though some talented folks still remain. It was painful to see the difference between 2 years back and now since there were so many highly talented, awesome people to be around. There are much fewer that remain.
Bonuses inconsistent / nonexistent, though perhaps that is a thing of the past. Promotions not really merit based and generally inconsistent.
Senior management has generally been pretty terrible in the past.