You can get basic rules for Lamentations of the Flame Princess free, along with a couple of adventures and supplements.

Pros:

- the basic system is a Basic D&D retroclone, so it will be both similar to and simpler than 5th edition D&D.
- the referee booklet is ranty, but it's good advice for running Old School games and horror in D&D, even if you opt not to use LotFP rules.
- the adventures are unlike those that you'd get elsewhere.
- the supplementary spells are unlike those you'd get elsewhere.
- both the adventures and spells are easily portable to 5th edition D&D if you feel like it.
- beyond the free stuff, LotFP has a large catalogue of additional material.

Cons:

- beyond the basic rules, it's all about giving the finger to mainstream ideas of good taste and what D&D is for. It's to mainstream D&D what gnarly Scandinavian Black Metal is to Imagine Dragons.