Here's a short summary of comments by engineers on Zubrin's engineering that you asked for, Mark:

Scott Lowther on Bob Zubrin's engineering
Space Sciences
Scott Lowther on Bob Zubrin's engineering
Posted by E.P.Grondine on 10/17/2002 2:18:20 AM
In Reply to: posted by Mark Eby on 10/16/2002 7:57:01 AM
"Christ, where does a guy start? Noting his unwillingness to listen to advice? His mismanagement of funds? His shooting off onto irrelevant, unproductive tangents while on a tight budget and schedule? His ability to distill a highly complex 6DOF situation down to a single back of the envelope 1D calculation basedon a single component, then looked surprised when it EXPLODES? His display of a lack of understanding of kinematics? His repeated reliance on "I've got to believe..." in lieu of "The numbers show..."? His lack of basic safety precautions when dealing with substances like blocks of lithium ("just cut it on the band saw"), silane and diborane? His un-nderstanding of the energetics of pressure vessels?

It's hard to know where to start."

See also:
http://www.thespacereview.com/article/677/1
For Dwayne Day's comments on underestimates of life support and other issues

Discussion:
http://forum.nasaspaceflight.com/index.php?topic=7253.0
in particular aerobraking, lack of artificial gravity, comments on solar power versus nuclear

A nice secondary collection of comments:
http://www.wired.com/wired/archive/9.07/mars_pr.html
From which one can locate the original comments, including Dr. Park's
Note Kim Stanley Robinson's comments on back contamination in particular

Martian dust, and problems with in-situ fuel production,
from someone who is no longer with us:
http://www.friends-partners.org/pipermail/fpspace/2007-March/022147.html

Zubrin as a social/political theorist:

For Bob's imaginary views of the current international political situation:
http://www.brothersjudd.com/index.cfm/fuseaction/reviews.detail/book_id/1342/

For the real drivers behind some peoples' fixation with manned Mars flight,
and Zubrin:
http://turnrow.ulm.edu/view.php?i=95&setcat=prose

BTW, I like Bob both as a person and visionary, but...

E.P.