Bug-fixing update 0.5.1 has finally hit the store. As usual, it is lagging a couple bug-fixes behind thanks to Apple slow-ass-ness, but you know the deal by now.
One important note: If you have been experiencing many crashes with 0.5.0, it is recommended you first delete the application from your iPod, before syncing again with iTunes (you can do this at any time and as many times as you want). This operation will unfortunately reset your KanjiBox stats, but it should greatly improve stability.