Let me start by saying: I became convinced over the course of my semester in Berlin that it's basically the best place in the world. It's a capital city, but without many of the things that ruin a capital city and make it boring and unlivable. It's cheap, young, political, and there's so much going on! It has amazing music coming through, really cool bars, beautiful and huge parks (so many for a large city!), and even multiple lakes not too far away to swim (some have real beaches). But in addition, the city also has so much history, including incredible museums, monuments, and memorials. All of this is not to say that Berlin is without problems, but you'll learn about those in your courses and become more attuned over time. None of it persuaded me to revise my opinion.
The IES Programme itself was also great. The people that work there were always available and helpful, whether with course questions or health insurance queries. I took half of my courses within the programme. I would especially recommend Multicultural Berlin with Sabine Berking-- I learned a lot about Berlin and Germany, and we took some awesome field trips to places around the city. (I also heard about many other trips from my friends in other courses.)
I would say that I did not feel hugely academically stretched by my IES courses, but they catered well to a wide range of German abilities, and if you are feeling more confident, there is an opportunity to take classes with ~real~ German students at Humboldt. This is something I would absolutely recommend. Though it was a little effort to make it work, it was definitely worth it to meet people outside the programme and feel like a real Berlin student. For this reason, if you are only going for one semester and it is an option for you, I would definitely recommend going in the spring, as the Humboldt semester doesn't line up particularly well with our semesters, and in the spring the extra couple months run into summer rather than your spring semester back in the US (I was in Berlin until the end of July). Plus it means that the weather only gets nicer and nicer while you are there!
Most of my friends in the programme lived in home-stays in the city, and it did seem that there was a real range in outcomes with that. I, personally, had an amazing experience and will probably meet up with my host mum whenever I return (hopefully soon). Others had more difficulties, which is tough for IES to solve, though they will talk things through with you. A couple other of my friends organised living arrangements outside of IES, and so lived alone or with other students, which is an option to consider if you're feeling very motivated.