Pros
Blocket provides an environment with great work-life balance, low stress and really nice colleagues. If you're looking for a stable 9-5, Blocket is the place for you. The fact that they're making money hand over fist doesn't hurt, as you can be reasonably sure they'll still be around in ten years. The benefits are fantastic, with lots of vacation time and good health insurance. The office is very centrally located and is quite nice - if perhaps a bit boring. Blocket is a great resume piece, as everyone in Sweden knows it and has used it. The company has a reputation of employing some really good engineers.
Kontras
Don't expect to learn a lot as an engineer. Blocket has its own programming language and a lot of technical debt that's not really being addressed by anyone (especially not by management). Projects that are internally perceived as "exciting" (for engineers) would be run of the mill projects outside the company. It's hard to criticize, as they seem to be doing something right, but there are no ideas being created inside Blocket that aren't first and foremost aimed at increasing advertisement revenue. User satisfaction and product quality is very low on the list of priorities. 9 out of 10 projects are some kind of integration with some other Schibsted company. This makes it hard to keep your motivation high and to feel proud of what you're doing. The compensation at Blocket is okay, but be sure to negotiate during the hiring process, because you're unlikely to see a substantial raise or any further negotiation after that.