I just used the LH business lounge in the B2X area (think it was near B22). My onward gate was B11, so I had to go through the wrong security line to get to B22, but they didn't give me any trouble with my B11 boarding pass. While the lounge was pretty crowded and the morning food not anything to be excited about, I though the shower experience was pretty good.

There was a sign up sheet. Took about 30 mins to have my name called (there were 3-4 people ahead of me on the list when I signed in). The shower room was very clean and modern looking, towels were clean and large (I hate tiny shower towels), and basic toiletries were provided.

Again, wasn't too thrilled about the lounge itself, but rather enjoyed the shower.

PS: One of the LH people told me there are also showers at the LH arrivals area that one can go out to and then clear customs to get back in (if that is an option for you visa wise), but they told me they thought the B22 showers were much better. I have no personal knowledge.