In flavor, it just doesn't have it. It has the same char flavor that a burger has... but without any meat flavor behind it, which is of course most of the flavor of a hamburger. Which makes sense, of course, because there's no meat. It's like picking up what you think is a Coke only when you taste it it's just sparkling water.
And then in texture it's quite different too. As if meat, instead of being ground (with all the satisfying texture that still has), were somehow blended into something more like a smooth paste before cooking. It isn't physically substantial or satisfying to chew, just mushy.
Since I'm not vegetarian, it's definitely not something I would ever order again because it doesn't have flavor and doesn't have texture. I'm not even talking about as a substitute, I'm just talking about as food period. Just give me some roasted veggies instead which have both flavor and texture. I mean, I love vegetables.
Or if you want a vegetarian "burger", a meaty Portobello mushroom is waaaay tastier.
But the Impossible Burger just feels like the worst of all worlds -- not satisfying like meat (just nowhere close), but also not satisfying as vegetables or any kind of "real food" at all.