The best jam I ever had was raspberry rhubarb

Rhubarb is actually really sour. As in, if you eat too much of it, your teeth will start feeling as if they're covered in fur, because it genuinely fucks with your enamel. (Rhubarb contains oxalic acid, which is also used in some tooth whitening products).

But it's basically never eaten without adding a boatload of sugar to it. So, you can kind of imagine it like those sour sweets, but stronger, and of course, it's a plant, so the taste is somewhat richer (although still not very rich for a plant).

As for eating it raw, well, then you can't really add sugar to it, so basically not palatable. I mean, you can do it, but unless you really like sour, it's just not good.

And it's only really similar to celery in terms of its texture and crunch. The taste is completely different.