It is certainly much easier to just screen cap or video tape you demoing your app hands on. I suggest doing this early to build buzz as it is quick and doesn't require much effort.
Appency is right though, it definitely takes a certain creativity to produce a good trailer.
Using
SimFinger,
SnapzProX, and iMovie I was able to produce the video below.
YouTube - TapnScrap Intro Video
I spent a ton of time on
Audio Jungle (my referral link) looking for just the right piece of music to set my video to.
I find it's much easier to edit your screen captured app footage if you have a tempo and a good piece of music behind it.
I am curious what the consensus is for adding voiceovers?