Usually we [me and my girlfriend] have to decide every day whether it will be a K-mount day or an M42 one (the M28/2.8 and M50/1.7 cover the wide-normal range, while the Takumar 50/1.4, Panagor 55/3 macro, Helios 58/2, Jupiter 135/3.5 and Takumar 200/4 cover the normal-long and close-focus range, in addition to a set of M42 extension tubes). So basically, if we need light, we decide upon K-mount, otherwise M42 is great fun
Obviously the M42 adapter is a must have, along with the AA battery adapter and a microfiber cloth.