The characters may seem flat, but that's just one perspective. I recently finished all three books and can attest that more nuanced, multi-dimensional characters do appear later in the series, though there are also even flatter ones as well, so perhaps it's a wash.
Highly recommend the audio book versions on Audible. Great narration.
Boomerang and Followup.cc are both great alternatives, and I use them too. I like the granularity, but the downside is you're sending them a copy of your email, which might contain sensitive info :\
Highly recommend the audio book versions on Audible. Great narration.