It's obvious that some people hate Salt and some love it. This is an effect of the way the book tells the story. The whole point is that you see the flaws of both sides and don't get a one-sided view of the events. This isn't a goodies-and-baddies book, and that seems to have been the objective. Since this is fairly new, it makes the book a fresh read, and I'm very glad that I read it.
Having said that, I found it difficult to get into at first. The reason for this is that I found myself hating both characters for their hypocrisies and dodgy reasoning that I'd just get annoyed with them and have to put the book down before it went flying out the window.
On the comparisons with Dune...? I don't get it. Well, yeah it has a heavy political component but its implementation is much simpler. Dune was a baroque galaxy-spanning feudal empire, and Salt is not. If you want another Dune, go and read Iain M Banks' The Algebraist.
I don't think this was trying to be like Dune, and the comparisons would have both Herbert and Roberts scratching their heads and frowning.
So, then, a decent book? Yep. For everyone? No.