At his blog Hercules and the Umpire, Richard G. Kopf, a judge on the U.S. District Court for the District of Nebraska, has a fascinating post on mandatory minimum sentences, which I would urge anyone who has been following the Canadian debate about them to read. (Indeed, this is the rare occasion on which you should read the discussion in the comments.) Judge Kopf is generally critical of mandatory minimums as a policy matter, but his views are nuanced. In particular, they call into question the argument against mandatory minimum sentences being made by the Québec bar in its challenge to the 94 mandatory minimums created by the so-called Safe Streets and Communities Act, SC 2012 c 1, better known as Bill C-10, which the Québec Court of Appeal recently refused to dismiss for lack of standing.
Judge Kopf’s post consists of his answers to a series of questions asked by a journalism student, the very first of which is the same as that posed by the Bar’s challenge:
are these laws an encroachment upon the judicial branch and the prerogative of the individual judge by the executive and legislative branches?
Judge Kopf’s answer is that
there is nothing inherently wrong with Congress enacting mandatory minimums. After all, Congress has the power to pick specific and definite sentences for any crime on the books.
At the same time, Judge Kopf points to a serious problem with mandatory minimum sentences: “[i]n order to maintain proportionality between offenders mandatory minimums tend to drive up sentences” imposed on those whose crimes are more serious than the least blameworthy ones that could be punished under the same offence, for which the minimum sentence should in fairness be reserved. In the United States, this happens through the intermediary of the Sentencing Commission which must, as Judge Kopf explains, “implement those minimums and then peg the rest of the sentences [provided by the Sentencing Guidelines, which the Commission develops] around those benchmarks.”
There are no Sentencing Guidelines in Canada, but Canadian courts have recognized this effect of mandatory minimums as well. In the recent decision in R. v. Holt, 2014 BCSC 2170, Justice Warren of the Supreme Court of British Columbia explained that
[s]ome mandatory minimum sentences have been found to create an “inflationary floor” that affects the sentence of not only those who might have received sentences below the mandatory minimum, but also those who would have received higher sentences, on the theory that the overriding sentencing principle of proportionality requires the minimum sentence to be reserved for the so-called “best offender.” 
As Justice Warren further explained, if the mandatory minimum for a given offence is in line with the lowest sentences already being handed to those found guilty of it, there will be no inflationary effect. But if it forces courts to increase the sentences at the lower end of the range for the offence, then it will also affect those offenders whose sentence ought to harsher.
All that to say, as the Nova Scotia Court of Appeal, as well yours truly, have already pointed out, that the big problem with mandatory minimums is not their effect on judicial discretion or separation of powers, but their effect on people being sentenced. And that effect, as Judge Kopf observes, can be very unfair. Judge Kopf acknowledges that mandatory minimums can be legislative response to disparities in sentencing for substantially similar crimes between judges and courts. They are, he says, “a way of imposing a minimum level of equality, albeit it at a great cost,” both that of the distortion of the sentences imposed across the board, and that of the injustice of punishments “that may have little or nothing to do with the proper sentence.”
Judge Kopf is no bleeding heart, and no libertarian, in case you’re wondering. Even if you think that Canadian judges and academics who have been denouncing mandatory minimums ― and, in the case of judges, striking them down on a regular basis ― are incorrigibly soft on crime, you should take what he has to say very seriously.