I recently installed Magic Lantern on my M and appreciated the improved ability of manual focussing, thanks to the focus peaking feature.
At almost-macro distance, the 100mm IS Macro L plus focus peaking gave me better result than using autofocus, so now I'm more confident about manual focus and I wonder whether an investment in "old" lenses (plus adapter) is worth or not.
With regard to one popular FD lens, I think that it is not worth to spend some 50 €/$ for an old FD 50mm F/1.8, because I can buy a new EF 50mm F/1.8 (which I could borrow to my daughter's 1100D/T3 or to my own 5D) for 90/100 €/$... but I'm almost sure that there are ancient lenses with better value for money than the 50mm F/1.8, or with unique features.
I don't own any legacy gear, but please feel free to post in this thread not only suggestions on possible purchases you made, but also your esperiences with lenses that you already owned and that woke up due to the arrival of a mirrorless body.
Thank you in advance for your advice, fellow friends from Canonrumors Forum!