Oh god, I can't rave about mine enough! I have the Oceans Motions 4-way and love it. I also can't rave enough about Paul @ Oceans Motions...his company gives the best customer service that I've ever experienced. He's always quick with calling back and he's very knowledgable.
Hey Gaby, the OM unit moves water through ports, redirecting water to different outlets depending on how it is set-up it can produce wonderful water movement randomly.
I have a 4 way with a deep sand bed and I did some mistakes not realizing its better to start it up without the drum installed then after everything settles a day or 2 later and after aquascaping is finalized you tweak anything needed then put in the drum . I say this because i called Paul several times as his 90 gallon flow diagram /setup idea seemed odd but it works good and he always either answered right then and there or very soon and he sent me a drum for sand beds so it wont get stuck in the drum overnight for free. Service is extremely good I bought directly from them and would go that route again
I'll reiterate what Plack said above: if you have a sandbed, ask Paul about getting the modified drum. Mine has jammed with sand particles several times. Not a big deal to clean it out, but kind of a pain nonetheless.