I’m not a huge fan of all the online prototyping tools that are coming out. Just like all the other wizbang whatever frameworks that are released or forked every other week, I feel like these are all the latest hype for newbies. Just one more layer of not being creative, and one more thing you’ll need to get your ideas out of to turn your project into a working site.
They say that prototyping can also make the design process more efficient and speedy. However, I’ve never had a problem just using photoshop, illustrator or for that matter sketching on a piece of paper. Seems like the web is finding solutions to problems that don’t exist.
I guess as web developer I just feel that there are easier ways to “rapid develop” than adding the complexity of these services. But, if there is someone above you telling you you need to use something like this, these are some of the better ones. Prototyping and wire-framing are important. Even though for the cost of these online services, you could get a subscription to the full Adobe Creative Suite and learn to to proper mock ups in Photoshop or Illustrator.