In reply to aln:
Glassdoor is the best for what you are looking for.
You still have to sift through and figure out which ones are deliberately placed by the employer to look great and which ones are closer to the truth.
I'd also recommend in the interview process, ask questions about company culture from the stakeholders you meet, in order to try to get a real insight into how it actually is to work there.
Culture eats strategy for breakfast... My own experiences in joining a new company have definitely reminded me of this...
Best of luck!