Flipbook vs. Flickbook? ...I guess we all love a little conflict. I use both, so here's my 2 cents.
Flickbook is basic. No undo, zoom, or brush size. You can't insert frames, or insert pictures/photos from outside. No cut and paste. You can't upload to a site or export to show off your animations outside of the iPhone. ...But what you can do, is make quick and fun little animations. I like it because it forces the animator to really think about motion, and not fret about anything else. It's quick. It's fun. ...It's definitely worth the $4.99 (unless you're a starving child in Africa).
Now Flipbook is ridiculously good. For $9.99 it is practically philanthropy. It has layers, color pallets, brush sizes, zoom in/zoom out, multiple undo's, cut and paste for frames, insertion of duplicate frames, import of pictures and photos from your iPhones photo gallery. It's remarkably easy to use considering everything it gives you. It has shortcuts to change brush size, do undo's, position the layer. It has onion skinning for the frame before, or after, or both (you set it as you like). It has an upload to a Flipbook.tv site so you can show your friends and family how much time you're wasting. It is an obscenely good app that will really make you open your eyes in seeing what an iPhone application is capable of. With a little practice it's actually quite easy to use. (...I love the repositioning you can do on a layer, because you can copy and paste an object to the next frame and move it just a little bit. I love a lot of things about it.)
Which one should you get? ...Both. Seriously, is $15.00 gonna break the bank??? Can't you almost find that in your sofa cushions? If you're into animation, you should really try both, because each creates a different experience when drawing animations. They're both worthy of purchase.
Flickbook is ok for beginners, but I can't say enough good things about Flipbook. For the upload capability alone, I can't see why anyone would want to be without FlipBook.