This article made me finally bite the bullet and convert. I've been using the new design for the last several hours. I happen to have a window open with the old design, so I know it's not merely my imagination that the new one is worse. Not enormously so, but definitely worse.
Read mails are more legible in the old version, because there is more contrast between black letters and the old light blue background than between black and the new gray.
It's also harder to parse the list of emails visually in the new design. In the old one, the 3D checkbox acted like a bullet point, and the name of the sender was closer to it. Now the heavy checkbox has been replaced by a faint square, and the sender's name is about 2x further away from it. So scanning my email is no longer like scanning a bulleted list. It's just rows of text.
That's a big deal functionally. There's a reason bulleted lists exist as a format, and removing the bullet points from the average bulleted list would make it significantly less legible.
It's a bummer to see Google making things work worse in order to make them look better (or worse still, more consistent). That's the sort of thing big companies do. Which I suppose Google now is. But they had at least been trying not to act like one before.