The non-mandatory nature of the sentencing guidelines has been emphasised in Spears v United States  USSC 21 January 2009.
In using the Guidelines to calculate the sentence for conspiracy to distribute cocaine, based on the quantity of drug involved in the particular offending, the District Court had held that the Guidelines yielded an excessive sentence. There was no other particular reason, arising from the facts of the case, to depart from the guidelines. However there were decisions of other courts which used criticisms of the Guidelines advanced by the Sentencing Commission as grounds for departing from the Guidelines. Referring to those, the District Court here imposed a sentence lower than the guideline range.
The Supreme Court upheld the District Court's approach:
" ... we now clarify that district courts are entitled to reject and vary categorically from the crack-cocaine Guidelines based on a policy disagreement with those Guidelines. Here, the District Court's choice of replacement ratio was based upon two well-reasoned decisions by other courts, which themselves reflected the Sentencing Commission's expert judgment ... See Perry, 389 F. Supp. 2d, at 307–308; Smith, 359 F. Supp. 2d, at 781–782; Report to Congress 106–107, App. A, pp. 3–6."
This is an application of Kimbrough v. United States, 552 U. S. ___ (2007) in which the Court had held that "under Booker, the cocaine Guidelines, like all other Guidelines, are advisory only".
The guidelines have been amended (Nov 2007) after the District Court's decision in this case. Here are the current guidelines applicable to this offending. These strike one as being complicated. In any event, the judge would have to decide on a sentence independently of the guidelines in order to be able to say whether he should follow them. There is really no substitute for an assessment of the particular offending in its context and in the light of sentences passed in like cases. The likely financial gain obtained or expected from the offending can usually be assessed and is a means of comparing cases.
Roberts CJ, dissenting and joined by Alito J, would have placed form over substance and would thereby have created injustice for the appellant, the rationale – if one can call it that - being that this was not a suitable time to answer a "novel question".