After thinking about one for nearly a year (and strategizing some Xmas/birthday gift cards). I finally bought a cover stitch machine!
I went with a Juki MCS1500, as it got the best reviews on Pattern Review and I could use my gift cards for it. I’ve loved my Juki serger and this is very, very similar for top threading–a bonus.
(Cat for scale)
A cover stitch’s primary function is to do that stretchy 2-row hem you see at the bottom of t-shirts, but it can do so much more! It can apply arm and neck binding like a champ for a super-professional finish on your gym tanks:
You can use the reverse side of the stitch to mimic the look of a commercial merrow stitch on your gym tights: 
(You do have to stitch the seam on a regular machine or serger first and then topstitch with the cover stitch, but it’s worth the time to look so fancy.)
You can even apply stretch lace and lingerie elastic! No more homemade-looking zigzags!
If you’re on the fence about one, I say go for it. Yes, I could do all these things with my fancy Bernina and my serger, but the cover stitch just makes it faster, easier, and nicer. As my dad says, “You need the proper tools for the job.”