Density is always a difficult comparison as it depends upon the area chosen. But,according to Wikipedia, Somerville is about 50% denser than Munich for example.
That's not really a direct comparison. Somerville is a residential area just outside of Boston, but central Boston is full of places similar to the German examples:
One of those pics is in the North End, an historically Italian neighborhood that has become increasingly primarily tourist-oriented with the removal of the above-ground highway. In this respect, it's very much like the old town of a lot of European cities.
Another in a somewhat grimy commercial street that used to be the Combat Zone/Red light district that nonetheless now has some very luxury high rise apartments on it.
The third is the high-end retail street in the Back Bay which is indeed a nice mixed use area so long as you have a couple million to drop on a condo.
To my way of thinking, Cambridge and Somerville are more livable though Somerville is mostly notable for not yet being quite as expensive as Cambridge for the most part. (Somerville was historically pretty down-market and the interest today is mostly because Cambridge has gotten so expensive.)
