So, just wondering: am I wasting my time (and theirs)? Should I desist or do you think it's occasionally worth doing for those who might read and begin to question? Have you ever had any luck on them perhaps?

As for 'luck', I find better 'luck' on doing discussion offline (face to face meeting).
I usually start by something simple like:
- Make them realize that "their religion/sect/cult" is more appropriately to be called Sunniism / Ahlu Sunnah Wal Jamaah
- Put forward an argument that Muhammad Ibnu Abdullah could not be a member or follower of Sunniism.
With the "Syahada" as the evidence.
Would Muhammad say something like "I bear witness that I AM the Prophet of God"??
As thus, Muhammad shall violate the "Five Pillar of Sunniism".
- I could also add that it is impossible for Abraham, Moses or Jesus to recite the "Sunnis Syahada" given Muhammad is not yet born during their lifetime.
And just stop there.. as people doesn't value other people opinion, people always value their own opinion.
And people need time to think it through by themselves..
Salam / Peace