It's expensive, but wasted time is also expensive. As soon as you run into the limitations of using Flash for mocking up your app within mockflow, you'll end up having to redo a lot of things which all takes time.
1. Create an account (30 secs)
2. Check my email (5 secs)
3. Press a "Click here to activate" button and be redirected (5 secs)
4. Wait for the Flash to load (5 secs)
5. Fill in my email and password (5 secs)
…quite frankly, you'd have lost me at step 1 normally. The only reason I signed up was to see how many steps it took to try it. Conclusion: too many.
The button on the homepage should say something like "Try now", which immediately launches the flash applet and lets me use it. When I want to save my mockup, that's when I want to enter my details.
The one hesitation I have with using tools like this "in the cloud" is the access its developers and administrators have to concepts. I understand the "release early, release often, tell everybody about your ideas" mantra, but my instinct still prevents me from dumping wireframes for a yet-to-be-released product in such an early stage onto such a site without some mechanism of protection wrapped around my content.
I really like the clean cuts and color scheme of the design, the sign up process could be better as @ashleyw states, I am currently testing 2 other wirefreaming web apps, and let me tell you, I have needed this (and no OmniGraffle Pro does not solve everything) for a while.
Looks promising but I did not receive confirmation e-mail :(
It's expensive, but wasted time is also expensive. As soon as you run into the limitations of using Flash for mocking up your app within mockflow, you'll end up having to redo a lot of things which all takes time.