AquaNox starts with a gritty dark underwater setting. I haven't seen this much (anyone know the game Deadly Tide?). It's a great atmosphere for this game.
The gameplay is basically like any flight simulator game. I think it's great and really throws you in the action, but doesn't stand up to the high bar of StarLancer (my favorite space sim ever). The storyline and missions are basically linear, leaving not a lot of room for free lancing, but you do get to buy weapons, items, and ships. This makes it sort of like the classic Crimson Skies.
All in all, AquaNox has a great story in a gritty setting and delivers one the great dog fighting flight simulation we all know and love. It's not revolutionary in any way, bit is a great excuse to dust off your old game pad or joystick. For $5.99 this is a great value.