Message I agree with? Well the message was that the bombing of Dresden was horrible. Can't disagree there. More largely, that war is horrible. Can't disagree there either. And I think it is important to remember the horror of it, and not think only in terms of abstract numbers. I can't say I do know much about the exact horrors of war (whether fire-bombing in Dresden or mustard gas in WWI (saw a picture of a victim on Quora yesterday)), and it is valuable to know. Not that I make decisions about people going to war or not, and not that I'll ever be in a position to :)
But of course, there's the other side. War is horrible, but avoiding war can be horrible too. The argument in Slaughterhouse 5 is that Dresden was unnecessary, a miscommunication, strategically meaningless and thus unjustified. And while it certainly is possible for something like Dresden to happen unnecessarily in the fog of war, and it's important to avoid that, the truth is, Dresden wasn't that simple. At least according to the Wikipedia article on it :), many of made the case for its strategic necessity. And further, while Slaughterhouse 5 claims the number of dead to exceed the deaths of Hiroshima and Nagasaki, in fact, the number was far lower, somewhere in the lower 10s of thousands. Not that that's really any better, but the point is, if Vonnegut gets it wrong there, then I don't put much faith in his analysis of the strategic advantages of Dresden.
Vonnegut was (apparently) writing from personal experience of the horror, and the book absolutely is able to viscerally portray the monstrousness of the bombing and of the horrors of war. But I assume this book is meant to be something more because 1) the bizarre plot of time travel and aliens would not seem to be necessary just to get across that message and 2) this book is considered a great work, so probably has more to offer? The message of the alien / time travel stuff is, I guess, that nothing matters in the long run, or that all of our fights, no matter the scale, are petty and meaningless? This is more my speculation, and I don't exactly have a PhD in this stuff, but let's discuss it for what it's worth. I of course don't agree that nothing matters at all just because it doesn't matter in the long run - we are here for a purpose. But it is true that we don't see the full picture and life would look very different if we did. Hey - if that's the point of this book, totally up my ally :) But I think the book sees different = meaningless and I see different = perfect. So not quite the same. Anyway, like I said, it's all speculation.
Bottom line, what this book gave me was a powerful perspective on the bombing of Dresden in particular and the horror of war in general. That doesn't make wars not worth fighting - on the contrary, remembering it and fighting the right wars is what keeps us human (how poetic). As far as any other deep meaning, I'll go read the wikipedia or cliff notes of the book now so I can actually get it :)
Verdict: 4/5 (only that high b/c it was good writing and short, do not take this as an endorsement)
Update: Oh it's about free will. A worthy conundrum.