It's charming, and reasonably climactic. In short, just fine. One feels that Nelson was so pleased with what a spectacular bit of programming modeling an Enigma Machine in Inform was that he forgot to ask whether it would be a good puzzle. If one more stecker were given, reducing the problem to brute-forcing one steckering and one wheel setting, it would be a less annoying puzzle. There are also a few wonderful puzzles.
Berlin is small, but tightly constructed and entertaining.
In fact, most of the worlds have tight and satisfying puzzles. It is arguably the finest piece of IF yet written.