I canāt comment 'cause Iāve never used these with software. I did use the a-192-2 with a YT vj program posted on another thread here, but other than that Iāve only used these modules with hardware.
All of the above. Note sequencing, chord progressions, triggers of samples, CC messages (so modulation), Program changes, MIDI clock. Targets? Hm, anything from obsolete standalone synthesizers such as Roland boxes, to VL70m or other physical modeling synths. Anything that can make a sound is a good target honestly.
If youāre in 12TET you will have zero problems. If you want microtonal there are workarounds, from either using external boxes like Midihub to create microtonal scales, to using pitch bend to make sure your notes are off-scale, to using synthesizers that have on-board microtonal capabilities which will repitch the incoming 12TET notes. In terms of CC control, if you know your MIDI you can mess around with LSB and MSB and get higher resolution than your typical 127 values that most people associate with MIDI.
Thatās an easy answer, definitely the SDS Digital Melisma, but I collaborated with SDS Digital on that module so itās kind of my baby and Iām very biased.
Other than that I worked on the M218 by Ladik, and beta tested on the ADDAC222 by ADDAC System, and the CV Thing by Befaco, and I find them all very useful and good candidates if youāre looking for CV-to-MIDI, but I also like the ADDAC221 and the VCMC which are modules I did not work on. I also own the C2M by mxmxmx and a-192-1 and a-192-2 by Doepfer, and there a few others which are worth mentioning (like Iungo from Entrospec) which never gathered much attention. It really depends what your end-goal is. Some are better at drums, some are very hands-on (VMCM, ADDAC222) so better for performance cases, others not so much (Doepfer), others are really nice for CC messages (ADDAC221) but not useful for notes, so itās relative to what you want to do, the space you have, and the complexity youāre willing to tackle.
If I only had to pick one Iād obviously go with the Melisma because it does stuff no other module can pull off, but I have the VCMC, Melisma, and CV Thing on the same case, so ā¦ that says a few things I guess 
I think your best bet is to figure out what it is you want to do and check the specs of each module to see if it fits your goal. Check number of channels, whether it does CC, chords, notes, etc, and whether the size fits your case. I think each design is pretty unique in a way, so thereās enough differences to make choosing between designs easier.
Iām happy that more CV-to-MIDI modules are out there. There werenāt enough choices a few years ago. General experience has been great, but you have companies like Befaco, SDS Digital, and ADDAC System who really respond to user feedback. I tend to push CV-to-MIDI to extremes so whether youāll be happy with incorporating MIDI in a modular setup is really personal - I embrace any issues or mistranslations that might come up, others are not so happy. But donāt get scared, you can get very precise results if you know what youāre doing, with not too much effort.
Usage? Sky is the limit, really.
