Efke is a thin emulsion, high silver content film. (I've been told that this line of films is the continuation of the very first thin emulsion films.) Efke 25 and 50 have a reduced sensitivity to the red end of the spectrum, which lends it a unique look. Efke 25 features extremely small grain. As with any slow film, it can be very contrasty.
Both from personal experience and from what I've read from others, it's a film that takes some practice to get "right", but the time spent is well worth it if you need a slow, extremely fine-grained film. Developed in Rodinal or another accutance developer, it can deliver extremely sharp negatives. Since the grain of Efke 25 is so small to begin with, a developer like Rodinal even at 1:25 does not lead to objectionable grain.
When you combine the reduced red senstivity and the very small grain, it is a unique film. It's not my favorite film for every day use, but I've spent some time over the last few months learning to work with it and I do find that it's a great film in the right circumstances. I especially like it in sheet film sizes for close-up work; it has a very smooth tonality that really matches my vision for such purposes.
(Efke 100, by the way, is a panchromatic film - it has good red sensitivity - and it has a tonality that I really like. It's my favorite landscape film in sheet sizes and one of my favorites in 120.)