Creating a professional looking screencast?
ScreenFlow lets you do all of that. You can split your recording into as many pieces as you'd like, add text boxes (which can be full-screen with a background or can be transparent on top of your video), and transition between pieces of video, text boxes, or even just graphics of your own creation. 

In a video I created for an app of mine in ScreenFlow I used Photoshop to create some still text screens in Photoshop (where I have super-picky-level control over the lettering) and then put them behind the app video capture. Here's what the timeline looks like in ScreenFlow:


You can see multiple tracks. The top one is my background music and the next is my voiceover, split up to allow for pauses while stuff happens on the screen. Below that is the text in the upper-right of the screen (you can see that I have it selected) with transitions from one screen of text to another, and then a graphic that shows the url for the app all of the time. Finally is the video track, which I've broken into pieces to allow for better flow.

The finished video shows all of it put together.