Yeah. Its easy to use, but hard to make something that actually looks good.
But my question is if are was some sort of library/collection of LibGDX particles.
Have you seen the tutorial vid?
I set skip to where Nate's making some nice fire-type effect. Looks alright for a few minutes effort.
Also dermetfan has his tutorial series, including libGDX particles: https://www.youtube.com/watch?v=LCLa-rgR_MA
As for a pre-made collection of effects, there are none that I know of. Maybe you should start one!
Edit: @Tom Strickland That looks beast. I may just have to buy that.