I liked Ender's game, and to a lesser extent Speaker for the Dead and Xenocide (for me, the actual Ender character becomes just a drag after the first book, and not all the others are compelling, but the quality of writing itself never falters). Children of the Mind got a little weird, but again, writing quality generally very good.
Ender's Shadow was a pure enjoyment to read. I read it long after Ender's Game and I knew OSC was a good author, and so I was able to savour it over the course of a couple of weeks in a way I rarely get to with other authors--because I knew what was coming next was going to be fun, which took the pressure off of wondering if the ending quality was going to match up to the beginning.
Shadow of the Hegemon was a different sort of story, but pretty near as fun, and starts getting into international politics on Earth--but also deals with the interpersonal relationships between the characters, and this is where the character of Peter really starts to take on a more complex and enjoyable cast. Shadow Puppets continues this, but as with the Ender quartet, books three and four are when the plot starts to falter a little.
The main criticism I have in the series (I don't mind the philosophical delvings, and they tend to be very relevant to the plot and characters) is the constant amount of snark from our "genius" main characters. It's all generally very clever, but therein lies the problem. At times it feels as if the characters are being needlessly hostile to each other just for the sake of making them sound clever. OSC is very good at witty dialogue, and it's fun to read. But sometimes his characters can end up sounding rather lacking in empathy as a result. I wish he'd been able to come up with a different way to ensure they all sound smart without being quite so...snippety about so many things.
Still, like I said, immensely fun to read.