Then use one of your friends that has the developer account, or wait till it's on every phone in the Apple Store. Either way, the point is to reserve judgment until you use it. Just like everything else.
You can absolutely pass valid judgement on quality of usability on sight alone.
If I showed you a picture of a 4 foot wide ship wheel and said I was going to install it in my Mazda Miata - you'd know instantly that it would be unusable, right? How about a screen-shot of a text editor that only shows you one giant character at a time?
Even as we move away from the ridiculous end of the spectrum of examples - you can know how usable something is based on past experience with similar environments.