Not really. Soylent is much cheaper than Ample, and that's a major plus for the cost conscious (which is a lot of people!)

As for the other ones - honestly mostly marketing I'd imagine. I've never heard of any product billed as a "nutritionally complete" meal. SlimFast to me sounds like a dieting product (as do weight watcher shakes), which isn't what I want at all. Non-dieting meal replacements, where they exist (I don't know the name of a single one!), just don't seem to target the same market that Soylent does.

Do other products exist that do what Soylent does? I don't doubt it. I've never heard of them, though.