Risk of Rain is a challenging cult classic rogue-like that unfortunately lacks the appeal a lot of other games in the genre have.
Its main mechanic is the difficulty that ramps up the longer you stay on a location. The risk versus reward system sounds enticing yet the execution of it severely limits your freedom of choice and forces you to take the same approach to be successful.
Freedom, that's what i found to be lacking the most in Risk of Rain. Contrary to the genre it belongs to, Risk of Rain doesn't allow you to decide your builds. You are force fed random items throughout the game and have no control over your builds.
Gameplay itself gets fun when you get overpowered but you spend most of your time slowly chipping away bullet sponge enemies, slowly exploring the world, dying a meaningless death then rinse and repeat.
In rogue-likes, each death should teach you something and make you want to keep playing. In Risk of Rain, i only felt discouraged. Even after completing the game i felt no accomplishment. Had no urge to continue playing which is crucial in rogue-like game.
I want to end my review on a positive note by mentioning the soundtracks. They are really good and worth checking out.
Bottom line is, Risk of Rain is just an average rogue-like with many minor and major negatives that are hard to dismiss. You won't miss out on much if you decide to skip it. You won't have a bad time playing it either so its up to you to decide if it's worth it.

It's good.
But buy Risk of Rain Returns instead.

Very enjoyable 2D platform game. The game uses a timer to scale difficulty which was a novel way build in a push-your-luck trade-off. Staying on the current stage to loot more stuff takes time, making the game harder, but each successive stage tends to have more complex enemies. Do you rush ahead to keep it easier and potentially end up ill-prepared, or do you loot every thing and risk the late game being too tough?

Interestingly, if you learn how to collect the best items quickly enough, then the difficulty scaling can actually kind of invert, helping you by spawning stronger enemies who drop better loot. It's possible to get to a point doing this that the game engine buckles under the load of your awesome panoply and the hundreds of enemies futilely throwing themselves at you — a state I consider a player achievement rather than an engine weakness.

Maybe my only gripe is how the story and setting feel pretty thin.

I think perhaps the most impressive aspect I've discovered was how I'd played probably 30 hours before finding out it uses the game maker engine, making it an excellent example of a pretty robust game made with tools very accessible to nonprofessionals.