The buildings don't have nearly the charm of what you would find in Paris, Rome, or much of London. There are some nice residential areas, some pretty tree-lined boulevards, some occasional 19th century (or earlier) masterworks, and scattered sleek contemporary successes, such as by Potsdamer Platz. There is lots of 1950s through 1980s mediocrity. There are nice river settings, but for the most part the city doesn't use its waterfront especially well. Many streets or plazas in the East remain huge cavernous monstrosities, devoid of Jane Jacobs-like inspiration.
It's nice enough that you can tell yourself it's not ugly, which perhaps is a sign of its ugliness.
I like that it's ugly, because it keeps the city empty and cheap and it keeps away the non-serious. There are not many (any?) splashy major sights. Even the Wall is mostly gone. The way to see and experience Berlin is to do things. The ugliness selects for people who want to enjoy the city's musical, theatrical, museum, and literary treasures.
Berlin is evidence that most tourists don't actually care so much about history, culture, and museums, as it is not for most people a major tourist destination, despite having world-class offerings in each of those areas. Mostly tourists like large, visually spectacular sites, or family activities, combined with the feeling that they are taking in culture or seeing something important.
There are, however, a fair number of Russian tourists who enjoy the nostalgic feeling they get walking through the eastern part of the city and visiting communist monuments and sites.
If you are in Mexico, and you visit ruins of any kind, prepare to see disproportionate numbers of Germans.