Civil War: Secret Missions is somewhat fun while it lasts, but that isn't very long, and there are better shooters.
It's great that we're finally seeing wars that aren't WWII or modern wars, and I love the setting, but the gameplay is generic shooter gameplay. Kill a bunch of enemies, disarm bombs, make stuff explode... you know, the usual. Besides an interesting set of weapons, there is really nothing fantastic about it. I'll go out and call it occasionally fun, but it's a bit of a bummer that the fun never really lasts.
What I mean by that is, the game is short. You can beat it in about 4 hours if you're looking for all of the extras, if you are just flying through the missions without worrying about side missions and collectibles, you can probably beat the game in about 3. In other words, you won't be enjoying too much of it. If you're an achievement whore, there's a chance you'll want to replay it on Hard and try to beat the time trials, but even that doesn't last long. The most you'll probably spend on this game is about 12 hours, and I highly doubt you'll put that much time into it.
Graphics are merely average. The game certainly looks better than a lot of other "meh" shooters out there, but if you're expect the gorgeous visuals of a Call of Duty or Brothers in Arms, you will be disappointed.
The sound aspect of the game is fairly good. The voice acting isn't awful, but it does have an issue that I thought games didn't have anymore... annoying repeating dialogue. If I heard "Nice shooting soldier!" or "Reload faster!" one more time, I was going to throw my controller at the TV. The soundtrack is great, it fits the mood well, and it could have certainly be worse. Most war games feature a pretty good soundtrack though, so you're not that special HC:CW!
Then comes the games major issue: No multiplayer. This isn't always a giant issue for games. Bioshock was fantastic, but that's because Bioshock featured a fantastic and completely replayable single player game. Sadly, HC:CW's single player was not even close to epic, and lacked any incentive to replay it more than a couple times.
My final thoughts? Well, if you want to play through the Civil War, HC:CW is one of your only options, and it's the most realistic one. However, if you are just looking for a good shooter, you could buy a lot better games.