Dr. Surendra Kumar of the Arya Samaj has written a concise and critical commentary of the "Manu Smriti". A scanned version can be located on-line via Google. In this commentary, Dr. Kumar states that spurious interpolations have been made to the original text of the "Manu Smriti" over time. The interpolated verses seem to be written in a different style of Sanskrit and they also tend to be "irrelevant and out of context but also thematically discordant". These interpolated verses were added to the original manuscript by various interpolaters throughout history who had vested interests in maintaining socio-economic and political power.
In his critical analysis, Dr. Kumar concludes that the original verses seem to embody Maharishi Manu as "a legal luminary and religiously righteous sage". These original verses embody "noble and just laws" of the time-period as well as displaying "due considerations to a man's potentialities, actions and abilities".
In brief, it seems as though the "Manu Smriti" discusses noble and well-balanced principles on the one hand, while other verses embody extremely objectionable principles (which seem quite anti-Manu in essence). These other verses also seem to be written in a different style of Sanskrit and seem out of context in the overall flow of the manuscript. Hence, the probability of interpolation of verses by vested groups is quite high.
In the end, I agree with Dr. Kumar's conclusions. Maharishi Manu was a stellar shastrakar and smritikar during the epoch of monarchy. I highly doubt that such a brilliant individual would include paradoxically contradicting verses within the same manuscript.
It is true that we cannot import all of the original verses of Maharishi Manu to our contemporary pluralistic society (founded on democratic principles). However, there is much within "Manu Smriti" which can be studied and emulated.
Dr. Surendra Kumar's arguments can be found in "Opposition to Manu: Why". It is published by the Arsh Sahitya Prachar Trust (Delhi). A copy of this concise booklet has been scanned online; one can find it easily via a Google search.
I'll end this response with a very relevant comment from Dr. Surendra Kumar (from the "Foreword" section of the booklet):
"No system in the world is completely foolproof or wholly acceptable. Even the present social and political system is not perfect. If some flaw creeps into the system then it can be remedied. Our ancient saints and sages have suggested to us a panacea for tackling such unwarranted situations:
[Sanskrit verse quoted from Taittiriya Upanishad, I-II-2]
This means that good actions of others should be embraced and imitated and not the rest."