Are the background sets available online good to work with?

Many of them are of excellent quality, highly detailed, virtually photo-realistic. I recommend taking a good look at the promo pics and gallery renders before purchasing an item, so you can judge for yourself.

Are they 3d (where I can put a character behind a building etc.) or jsut one dimensional...

Backdrops and backgrounds (DAZ's Cyclorama, nerd's backdrop kit, etc.) are somewhat limited, though they allow you to pose your actors in front of photographic environments. Other sets are truly three-dimensional, can be viewed from almost any angle, and permit full interaction with your actors. stonemason's environments at DAZ are good examples of the latter.

PS - does anyone know where I can view CGI movies made with Poser/(any other software)??

Renderosity's Animation Outlet.