I wanted to see how it worked, here's my thought process:

  * How does this work?
  * Hmm, no, I don't want to login or signup.
  * Oh, there's a small demo button.
  * Hmmm, I have to sign in?
  * *Sigh*, okay. Click.
  * Hmmm, a login page, this is too much trouble.
  * Leave.

Granted I wasn't too invested in the product but I recall there was one site (that "high-res" mockups one) that got promoted here recently that allowed me to start playing right away without signing in/creating an account.

Obviously this needs to be balanced against people who might build something and then lose it if they don't realise they have to sign-up to save it.