I’ve been doing a little of both buying and building synth modules. I’m not very good at it yet, but getting better by the module. Just finished a saw animator module, just started a ring mod/noise module and I’m about half way through building a MIDI to CV converter. But, I’ve purchased quite a few dotCom 5U/MU modules as well as some other custom stuff like EGs with built in VCAs (I actually did the VCA mod, but bought the kits to do them) and an awesome RCD/SCM module.
Modular freakin rocks!