I just finished reading Yours Truly, Lucy B. Parker: Girl vs. Superstar by Robin Palmer (#93 in my hundred book challenge) and really enjoyed it. What made it so fresh and enjoyable was definitely the main character Lucy, with her so-true to life grade 6 concerns and her personality quirks and anxieties. The novel is a great example of a book with a strong main character voice, which is really what got me hooked on reading it. And it was fun!
It got me thinking a little bit about the issue of believability in middle/tween books though. Some (adult) reviews of the book mentioned the unlikelihood of Lucy (the main character's mom) getting involved with the Dad of a tween superstar [this is a key element of the plot]. But I wondered whether that would matter as much to middle grade readers.
To what extent do middle grade readers accept a premise, without questioning whether it would happen in real life? I don't know the answer to this, but I suspect it has a lot to do with:
a) how engaging the characters and the voice are
b) background knowledge or experiences that relate to the premise (e.g. if you know a lot about it, you're more likely to be irritated when it's not realistic or logical)
c) the way it is presented (My 15-year-old points out that it is unbelievable that there are wizards, but that the Harry Potter books are written in a way that makes them highly believable.)