Wonderland is and will probably always be my favorite fictional world (at least, I think it's fictional. Whoever can bring me there however is always welcome to send me an invite!) and Splintered will always be one of my favorite retellings taking place in this world, especially because the Wonderland Howard created is so incredibly beautiful, dark, dangerous, unique and insane. This story has been on my shelf (in the paperback of Stain) for quite a while already and I finally read it!
It's a really nice story that doesn't particularly add something really important to the already existing world, but that does contain everything a Wonderland story needs to have. It has some really intense emotions while Morpheus is waking up from his transition into the moth we get to know him as. We meet a few really nice and familiar faces again (well, technically for the first time, since this is a prequel). And we once more get to travel to the wonderful world of Wonderland and we're meeting some more unique creatures and we get to see some more of the unique magic.
Not an absolute must read to understand or appreciate the series, but an absolute must read if you do love Morpheus and Howard's Wonderland as much as I do.