This is a very strong 3.5, but I ain't rounding it up, and I'll tell you why.
Moore mentions in the foreword how this story was really more of a fun collaboration between him and Ian Corson. How the story itself was more visual when he thought of it so it wouldn't work as a novel [what with the writing style Moore has] and Corson admits that as a movie it probably wouldn't get made [and cost way too much to make] but it would be something fun to write, so they did.
Now I gotta admit, most of the time when I read a book by Moore I see the scenes as comic book pages in my head, because to me, he has that sort of writing style where stuff feels like it should be in a comic. You just gotta match the story to the proper art style and you'll have something great. This story definitely belongs in a comic medium, but it could have been better. I'm also sort of judging it on the fact that it was a movie script first which got turned into a comic, and I feel this greatly affected how it was made.
First of all, films can have any number of transitions between scenes, and they can be really quick, especially in action films [like this was supposed to be]. Unfortunately, a quick back and forth between 2 different settings with different characters can be done quickly and make sense. But in a comic you need proper transitions between scenes or that shift can be very confusing for a reader. I'm not saying I didn't understand that a scene was changing, I'm saying that when you're reading a comic you need a fluid transition to help keep you immersed in the story. If it's almost a full page of one scene and then suddenly a panel leads to a different scene and a different character it's gunna be a little jarring, and can easily push you out of being immersed in the book. Which is what happened several times during Griff. As a movie the quick scene changes with lack of obvious transitions you could have gotten away with, but not in the book.
Secondly, movies have to move quicker, they've got like 2 hours [maybe] to get a full story out to you, it's not ok but more understandable if the characterization in a film is a little more implied than shown. But with a comic that's roughly 200 pages I expect a little more in the character department. You can actually round the characters out, make them feel like real people; and there were some attempts made; moments of emotional bonding and coming together. But at the end they still felt like the same cookie cutter characters we met at the beginning of the story, goofy skateboard dude, hot video-game gun chick, animal lady, army guy, like yeah I know more about them, but they don't actually feel like people to me [and I know yer sitting there all "But they ain't people tho", well shut up, I know that, but I could at least feel like they're trying to be real people. I've read plenty of comics where my brain went this feels like a well rounded human, and it's a drawing on a page, so it is possible].
Thirdly, and this is my complaint with any movie based graphic novel adaptation [and I feel like it applies even if the film didn't get made], the story felt very rushed and it was more like a summary of what happened than actually being involved on the journey. Once again, you can get away with this if your movie characters are rushing form place to place, and the switching between scenes is more to hide how much time has passed as they make their way to the target zone. The audience knows time has passed, even if we don't know how much [though if we're heading to Florida from New York [even with submarine and tank] it's still gunna take a long while [something along the lines of 2 months if you can mostly move by night]. A movie can make this feel like no time at all. But a comic can really stretch it out, make you feel those 2 months, build the characters during that time, build the plot up [though Moore really admits to there not being a plot, which also affects how immersive the story can be] get people more invested in the story they're reading. Cause while I found it fun, I probably won't think too much about it, I might not even remember what it's about a week from now, and it's honestly cause it didn't give me much, even in the terms of action and thrilling scenes they were over really quick, and a few weren't really clear on how shit was going down.
It was good, but felt rushed and a little empty [due to no real solid plot going down], and I feel like we really could have done more with it and had it be awesome. But if you're looking for just a quick story about aliens and attacking giant monsters to save the world, this is probably up your alley. And Moore if you wanna do anymore comics, hit me up.