Like many of the other Flashpoint tie in issues from DC Comics, this issue has literally next to nothing to do with the Superman which currently exists in DC continuity -- that's not to say this book isn't interesting.
Take everything you know about Superman, and turn it on its head. This issue has nearly nothing to do with Superman, and is predominantly setting the reader up for what's to come. In fact, there is no appearance of Superman in 99% of this issue. The story opens with Lieutenant Sinclair who's been recruited by General Lane (Lois' father) to undergo experimentation in order to make him stronger. Think Super Soldier serum only gone terribly wrong. Sinclair acquires many of Superman's abilities, and while the reader knows something is wrong with him, it's still hard to tell what it is because the entire issue is told through his perspective. The fact that you only see the story from his point of view will make you wonder what's wrong with him; it's also what holds your attention.
I won't complain about Superman not being in this issue very much because this issue is predominantly set up for the story to come. My only complaint is that without a lot of background information on Sinclair's character, it's hard to get invested in the character and to care about what happens to him.
This issue wasn't bad by any means; in fact it's one of the better DC Flashpoint tie in books that we've seen, I just didn't love it. I am, however intrigued by what happens towards the end and am definitely looking forward to seeing what happens to Superman in issue two.