Not pretty but full of culture
In our mixed group of friends Berlin was controversial. Some, including myself, loved it, while others found it boring. It's much more than just a sightseeing city because it's a living and breathing place full of interesting and different people, and it's only just growing from its recent history unlike very historical and static cities. It's neither particularly clean or pretty but actually that only adds to its unique story. Great nightlife if you prefer it more 'raw' than in most places.
Insider Tip:Register for free outside the Bundestag (parliament building) to visit the dome above it. Great views, but also a symbolically important place to visit to really understand Berlin's past and it's new direction. Free audio guide included.
The Story of Berlin museum is around €10 but well worth visiting if you have up to 2 hours spare and want to learn much more about the city's history. Quite interactive and theatrical.