Right now there is no good way to create an automated mirror of several parts in an assembly. The current "Mirror" command really just creates a mirrored Derived part (or a mirrored assembly) for parts or assemblies which cannot be made symmetric by simply being re-oriented. This is useful but it's not what most people want when trying to perform a "Mirror" operation in an assembly.
The mirror command does not maintain the positional symmetric relationship between the original and mirrored parts in an assembly. It should. If I change the position of the original part, the mirrored part should reflect this change on its side of the mirror plane.
Just like with a Pattern, there should be no constraints involved. I should simply have to pick the components to mirror, pick the mirror plane, and Inventor should look at each part, and essentially ROTATE it about the appropriate axis and stick it on the other side of the mirror plane. If there are sub-assemblies or parts that can't be made symmetric about the plane by simply being rotated, then Inventor should give me the option to do what the Mirror command currently does as a part of the Mirror operation. But then the Mirror operation should appear in the browser tree as a "Mirror pattern" which can be modified later, and which will maintain symmetry between the original and mirrored instances.
Note: I'm aware that there are some instances when a 180-degree circular pattern will accomplish this, but that doesn't always work. If I want a left/right mirror of several components, a 180-degree circular pattern about the vertical axis will only create that symmetry if the components are also symmetric on the forward and rearward sides of the rotation axis. If they're not, then the differences between the front and back will be flipped on the mirrored side. Only a true "mirror pattern" will create left/right symmetry in this case.