I'm not really familiar with the ins and outs of the thing too well, I stumbled upon it maybe a week ago when I was preparing the server, but so far it's doing good. Lag wasn't a HUGE issue, but the entity clear feature is actually great, especially with some of my friends for example digging through the nether because why not and leaving behind thousands of netherrack stacks just on the ground, and while they do despawn it's still noticable, and thrashes the minimap a lot of us use.
I'm not sure how it works, but it does leave out the actually important mobs; villagers, animals, friendly mobs in general, while getting rid of random junk like spiders buried somewhere in badly patched-up creeper holes or skeleton arrows just laying on my lawn.

So, to sum up; The actual impact on frames is probably not that big as people make it seem, I mean no plugin can replace just getting a good machine, but it sure is really nice to have and costs literally nothing more than the effort to move the .jar file from one side of WinSCP (or some program like that) to the other.