Now let me go ahead and get this on the table. Lost in Nightmares is radically different from the rest of RE5. It's probably more co-op oriented than the rest of the game combined, for one thing. It's also focused much less on action, and more on the survival horror elements the series became loved for. In fact, it's generally smart to flee from what few enemies you encounter, rather than fight them, since ammo is scarce and your foes are tough. Basically, this is old-school Resident Evil gameplay, with beefed up graphics and co-op play. If that sounds good to you, I don't think you need this post to decide whether or not it's worth the $5.
In truth, the game is composed of only 3 major parts. The mansion, and two different underground parts, then a brief battle with Wesker. The mansion is littered with throwbacks to older RE games, from the way you open doors, to the minor scares and eery atmosphere. You can even try out the old school camera angle by trying to leave through the front door.
As you delve deeper, you encounter several sub-bosses. While it's no doubt possible to take them down, the mazelike nature of the pathways that you need to navigate to outmaneuver them to hit them from behind (where their weakspot is) makes attempting it a bit foolhardy. So you'll almost never fire a weapon in the game, except to hit score pieces (which replace the BSAA emblems). But whether you choose to make a stand or flee, tight coordination is necessary to survive this DLC. Even the Wesker battle is tougher, with there being no place to hide, and many of his more crippling moves being avoidable only with a partner to help you out.
If you loved the more action oriented nature of RE5, this DLC may not be for you. Instead, stay tuned for the next piece of content, Desperate Escape (which judging by its name and trophies, will be much more of an adrenaline fest) due out in a few weeks. But if you were disappointed with how different the game was from the rest of the series, you're in luck, because this DLC was made just for you. While I loved RE5 for what it was, having not played any previous Resident Evil games, I was thoroughly impressed by this new style of gameplay. And at $5, it's really not a bad value. Here's hoping RE6 is more like this. 8.5/10.