I used Fireworks, Lumzy, Justinmind and Maqetta for mockup.
Fireworks is a great tool for any kind of design job unless you need professional vector or photoshop editing. You can easily find mockup libraries and create links, connect pages, even add states on the components. However, you can not achieve advanced gui component behaviors such as datagrid, accordion etc. Learning takes little bit time but it is worth.
Lumzy was great in the beginning. I could so easily create interactive mockups with very rich gui library. You can even add your team mates into the lumzy project and work together on the same mockup through your web browser. The best part is it is totally free. However, I started to get strange errors once the number of mockup files increased. In the end, I was not even able to save my project. So, it is probably the best free tool for simple mockups but not a complete solution.
Justinmind is an expensive tool. It is very professional and detailed. You can have the feeling that you are designing the actual product with it. However, it sometimes might be too much for simple things. I was advised to create the whole accordion component by myself just to add one more slice into it. After spending a few hours, I gave up since it does not make sense at all to waste that much time on a mockup element.
If you wanna continue over your mockup to actual design then you can give a try to Maqetta. It is free and enable you easily place html/javascript elements in many layouts including ipad and iphone. However, it requires far more effort than simple mockup process as you can guess.
I think there is still need for a complete mockup solution, which will be cheap/very easy to use/rich of gui elements/extendable and allow you continue your actual design on that mockup.
