Optically, the Jena Flektagon 35mm 2.4 is a very good lens, whether it's used with a normal adapter or with a speed booster. Surprisingly good sharpness for a vintage 35mm lens and it has the ability to get really close to the subject. However, its weakness is that it can have sticky aperture blades. Many Flektagons suffer from it, including mine. An alternative would be the Pentacon 29mm 2.8, which is much cheaper, certainly when you live in Europe.
X-T1/Pentacon 29mm 2.8/and probably Lens Turbo II
X-T1/Flektogon 35mm 2.4/normal adapter