I've had the Impossible Whopper (tried out of sheer curiosity) and the Beyond Burger (friend took me out to lunch and she's vegetarian, so I didn't want to make her pay for meat).
Overall, I thought the Beyond burger was pretty bad. It had an overwhelmingly bad odor (smelled like ... boiled peas) and the flavor was nowhere near actual beef. Very, very dry, too. I'd never have one again.
The Impossible Whopper was ... something. It smelled like a beef burger, it looked like a beef burger, and for the most part, it tasted like a beef burger (I'm guessing the high sodium content has something to do with it). But again, the texture was off, almost crumbly. But it wasn't a waste of four bucks, I suppose. Not something I'd ever go out of my way for, either.
That being said, I'm glad for the surge of improvements in plant-based meat substitutes. Our meat consumption, not just as a country but across the entire world, is entirely unsustainable and is one of the biggest contributors to climate change. The technology will only get better as time goes by.
It's easier if you don't really think of them as being hamberger. Just think of it as a food that vegitarian a can eat that gives them complete protein if eaten with bread.
The shape is just an option that is something most people are familiar with.