ShadowGuard tells the story of you, the only child of a city magistrate. When the module opens, you find yourself graduating from the local academy, which means you've just become a citizen of the empire. One of the (perks) of being a citizen is that you get to sit in judgment of criminals, and so your first task in the module is to decide the fate of some guy named Yakir (the Black Butcher.) Now, this isn't a case where you might find Yakir innocent if you dig deep enough; he's an evil guy, deserving of his nickname, and so it's merely a question of which punishment you want to give him. However, your graduating class includes two other people, and of course they're split on which punishment Yakir should receive, and so I thought this was going to be a cool way of deciding which of your classmates was going to be your henchman for the module. But, alas, there is little that could be considered (cool) about ShadowGuard. The fate of Yakir is completely meaningless, your classmates pretty much disappear from the module after the graduation ceremony, and the graduation ceremony itself isn't even necessary.
I think if you were to go to a website that hosts fan-created modules, and if you were to download ShadowGuard from there, you'd be pretty happy with it. It works, the rules of spelling and grammar are largely followed, and there's enough going on to keep it interesting enough to play. It's just that, as a pay module, even if the payment is only $5, ShadowGuard seems sort of weak, especially when compared to Kingmaker. There is almost no voice acting, the module is short and linear, and there is little going on that allows you to actually role-play. For example, at one point somebody gets kidnapped. You get a dialogue option that essentially says, (Screw him. He deserved it.) But then it turns out you have to rescue the guy to advance the plot, and when you do, your alignment shifts 10 points towards good! So much for being evil.
There are also some balance issues. The module creator seems to have a high opinion of what level 3-5 characters can do (which is where you're going to find yourself in the module), and one of the first battles in the module pits you against six bandits, all of whom can sneak attack. That's a recipe for dying a lot, except for 1) you get a henchman for the battle (the henchmen in the module are temporary, and only show up for specific quests), and 2) henchmen can't be killed! I found that fact to be sort of annoying, because it took all strategy and tension out of the battles. I could just send in my henchman in to battle everybody while my character maybe picked off one or two if I got bored. Yay.
So I wouldn't really recommend ShadowGuard, unless you're just desperate for a Neverwinter Nights module to play. Even with its 2-4 hour playing time, I struggled to find enough enthusiasm to finish it a second time to see if I'd missed anything interesting (I hadn't). The only way its purchase might be worthwhile is if the version of Witch's Wake included with it is a lot of fun to play.