It's a good DLC, brings two extra chapters, the Lost In Nightmares chapter is very good (it's just a bit lacking in the section where you temporarily lose your weapons) and the Desperate Escape chapter is like the war section of Resident Evil 4. But the chapters are short and you don't carry over your campaign progress.
The Mercenaries Reunion mode is kind of unnecessary, the new characters are really cool but I think it would be more appropriate if they were incorporated into the original The Mercenaries like it is in the PS4 and Xbox One versions of the game with The Mercenaries United.
The online Versus mode is more like an extra of the DLC, it's currently abandoned and when there are people online, they're boosting. And speaking of the Versus mode boost, you have to grind a lot. In the PS4 and Xbox One versions the grind for the Versus Mode boost is more reduced.
This DLC also brings new outfits for Chris and Sheva, they're really cool outfits.
If you like Resident Evil 5, I recommend buying this DLC. If you think it's not worth it because of the few hours of gameplay it will add, you can buy it when it's on sale. The frequency of sales for this DLC has decreased because it's currently no longer possible to buy only the base Resident Evil 5 on Steam, but you can have the Augmented Steam extension installed on your browser and when you view the game's Steam store page, it will inform you when games keys reseller sites have the DLC on sale. Then you can buy it through these sites and activate the key on your Steam account. The sites this extension recommends are safe.