as far as i can see,
eyon fusion is more for compositing...
after effects is more for effects...
premiere is for cutting and editing...
speed edit is for fast cutting and editing...
We've been using Fusion for fx work for the 10 years or so I've been with the Box at R+H Studios, and it was used long before I got there. These days, with all the passes we can embed in .exr files, we've been using Fusion for motion blur, dof fx, fog, refraction, and in many cases we can relight and enhance the lighting in a scene. Of course we also do many particle fx (including some gooey liquid and smoke fx,) as well as 2 1/2D tracking, volumetric rays, and all the usual image enhancing, chromakeying, roto-scoping and wire-removal stuff you would expect to be able do with a compositing package. Because it's nodal, it's much more flexible and efficient than AE's layers based system, and in recent versions it's started to leverage the GPU more for speed. Oh, yeah, it's also compatible with most AE plug-ins, so in many cases you have the best of both worlds. Also, the program is very scriptable and customizable; we been able to add tons of in-house plug-ins and enhancements to our Fusion pipeline. Being able to import LightWave cameras and scenes is another plus.
I've preferred using Fusion for a long time, but I feel it's becoming too expensive to maintain, especially for an 'indy' artist. (I have two personal licenses which I may not bother upgrade further because it will cost me about $1300 to get them up to date. Considering I only do one or two non-commercial short subjects a year with Fusion, it's probably not worth upgrading anymore. Sigh!)
That said, there are some very interesting 3D plug-ins for AE I wish we could use which are not compatible with Fusion.
For editing, we use Velocity HD. Personally I like Vegas Pro.
G.