Intellicad-Autocad compatibility

Hi Folks,
My apologies in advance as I'm sure this sort of question has come up
before. I haven't used Autocad since about R14 and will have to get back
to
it soon. But rather than the latest Autocad, which will no doubt still be
extremely pricey (for my pocket),  I'm thinking of looking at one of the
versions of Intellicad.

I've heard that Briscad and Cadopia do a good job with compatibility and
are
reasonably priced, but would like to know the differences between these
two
and how each rates in the compatibility stakes with big brother Autocad.
Are
any differences so great as to make a drawing non-usable/viewable if it
has
to be amended/viewed in Autocad? Any other comments about Autocad &
various
versions of Intellicad would be most welcome. How about the 3D aspect?

My area is mostly mechanical design with a heavy dose of plant layout when
required. Occasionally I get into 2D architectural stuff, but not often.

Many thanks,
KerryC