So many preschool and kindergarten apps teach reading by sight words, and even those who teach "phonics" jump around with no discernible scope and sequence. These apps are clearly well-thought out, and my son absolutely loves them. He will be 3 and a half in August, but he is already reading and spelling most of the level 2 words. Please, please, please Preschool University, put out more apps, because everything from the quality to the educational methodology rocks. And they're free? I can't recommend these enough. I would only wish for you to make math apps!
What makes the spelling apps so helpful is that they allow little ones like my son who is reading but can't yet control a pencil or pen to write to practice encoding words, as well as decoding them in the reading app. Many reading instruction methods agree that spelling reinforces reading, and this allows him to practice that quickly and easily.
I have various reading instruction books at home, including a complete Hooked on Phonics set, and we hadn't really touched them because I didn't want to rush my son into reading - well with these apps and a teensy bit of help from Mama, he is rushing far ahead of where I thought he'd be. I'm not embarrassed to admit that my iPhone and iPad are teaching my son to read. ;)