Yes, by all standards, it’s safe to say that this may be of the greatest Stadtbahn/light rail systems of the world, and potentially the largest. With nearly 11 lines and 222 stations and nearly 507K riders per day, it has nearly twice the ridership of Boston’s Green Line and nearly the same ridership as the entire MBTA network, and it also delivers relatively penetrative levels of coverage to the city of Cologne. One could argue that it’s a 1960s advancement of an idea that we in the United States pioneered at the turn of the 20th century, but allowed to languish until MUNI’s decision to run their streetcars into the Market Street Tunnel in the 1970s.
The network itself is a mix of pure street running to segregated street ROW to private ROW that can pass into some rather lightly populated areas with trackage that is shared with local freight lines. The lines themselves are divided into a low floor and high floor network that is a legacy of the old streetcar oriented focus of the lines in which some lines were upgraded with high floors and other lines simply weren’t. The high floor network sees Stadtbahnwagen B and K5000 stock while the low floor network sees K4000 and K4500 stock, and both types of stock tend to run in two car formations. Maximum speed, IIRC, is 80 km/h (50 mph) which is typical of many European systems, but the rolling stock has great acceleration.
From a railfan perspective, Koeln can end up poisoning an American railfan. While we have numerous light rail systems in the United States, none can truly compare with the experience of the Cologne network. High frequency service on certain trunk lines approaching 30 tph, numerous lines with high urban densities, and a mix of ROW styles creates a very interesting and fun to ride network, along with a usable product for the people of the Rhine-Sieg region. Even Portland, the poster child for American light rail systems falls a little flat in comparison. Mind you, I’d argue that Frankfurt’s approach to the Stadtbahn concept is probably what influenced American design more so that Cologne.
As for the city itself, it’s a nice, relatively compact and tourist friendly city. It’s larger and a lot livelier than Frankfurt, but I’d imagine that New Yorks may find the city to be relatively “cute” and small. It’s definitely worth spending a few nights here not just for the sake of riding the Stadtbahn, but for exposure to one of German’s regional and Europe’s media and trading hubs.