Well, it doesn't taste so much like cabbage. I'm not sure what I would say it tasted like. Reminded me a little bit of asparagus but that might just have been the texture of the stalk. Someone online compared the taste to spinach.
The stalk is more skinny and doesn't look like celery the way bok choy does.
Edit: I just cooked some more of the yu choy and I notice now that the stalk does still resemble celery in places, but slender celery. Doesn't open up wide at the bottom. And all this is silly -- how much it looks like celery or doesn't -- since it doesn't taste like celery. ... but celery scares me, so that's maybe why I care about that part.