Judicial decision making, for example, in the course of judicial review of statutes, is less partisan than political decisions by a large margin (empirically), even though many cases have a partisan angle, but it is also not free of partisan influence.
Some issues decided by courts are handled in a more partisan fashion than others.
For example (citing statistics I gathered from primary sources in 2006):
[R]oughly 84% of [U.S.] Court of Appeal decisions (a sample year) are
unanimous. This is consistent with a finding that 71% of U.S. Supreme
Court decisions were unanimous from 1889 to 1959, that in a recent
sample year 87% of California intermediate court of appeals decisions
were unanimous, and that in the Missouri Supreme Court unanimous
decisions are reached 98.5% of the time. Thus, while partisanship
matters and influences results even in non-unanimous decisions, there
is wide consensus on many legal issues.
About 91% of cases in the Colorado Court of Appeals are resolved with
unpublished opinions or dismissals, which are generally unanimous. In
addition, a large share of the 9% of Colorado Court of Appeals cases
that are decided with published opinions are unanimous. Good
statistics on the exact number of non-unanimouos published opinions
are unavailable, but it is safe to say that the number probably less
than a third of all cases. In other words, less than 3% of cases
appealed to the Colorado Court of Appeals are decided on a
non-unanimous basis.
Somewhere between 65%-75% of Colorado Supreme Court decisions are
unanimous (it is hard to provide an exact percentage with certainty,
as the only statistics that are easily available on the total number
of dissenting opinions and some cases have more than one dissenting
opinion), which is not too surprising because the state supreme court
handles only the hardest cases.
It is also important not to exaggerate the role of the U.S. Supreme Court in judicial review which is a power held by a U.S. courts from a lowly limited jurisdiction village court to a state supreme court or U.S. Court of Appeals. Decisions on constitutionality are always made in the first instance in trial courts and the U.S. Supreme Court is only vary rarely the last word in a case and only rarely has any role in a case.
Appealing parties may ask the entire Court of Appeals in a circuit to
second guess the decision of an individual panel (or in the 9th
Circuit, a much larger panel of the court rather than the entire
court). This is called an "en banc" review of a decision. Thus, a case
can be appealled from a trial court to a three judge panel of the
Court of Appeals, and then receive "en banc" review, and then go to
the U.S. Supreme Court. [The en banc review step is optional.] En banc
review, like U.S. Supreme Court review of a Court of Appeals decision,
is discretionary and rarely granted. . . .
In 2002 in the 9th Circuit, for example, 801 published opinions were
issued out of the thousands of cases that came before the court, of
which about 40 were considered for en banc review, and 18 received en
banc review (which in the 9th Circuit is actually a large panel of the
court rather than the entire court). This was just 2% of cases heard
on the merits.
En banc review is less frequent in absolute numbers, since the overall
caseload is smaller, in other circuits. In 1999, a fairly typical
year, only 94 cases in the entire federal system receive en banc
review, which makes this level of review similar in frequency to, if
not less common than, U.S. Supreme Court review. The vast majority of
Court of Appeals cases decided on the merits (probably about 95%
nationwide) are never even considered for en banc review.
Less than one case in a thousand filed in a U.S. Court of Appeals is
reversed in en banc review or in review by the U.S. Supreme Court.
Colorado Appellate Courts often get the last word, as well. In the
most recent fiscal year for which figures are available (2005-2006)
there were 2,622 appeals concluded in the Colorado Court of Appeals. In
that same year 868 petitions in certioriari (i.e. appeals from
Colorado Court of Appeals cases) where made to the Colorado Supreme
Court, and 78 cases were decided on the merits with written opinions
(a few of which would be in original proceedings direct from trial
courts, such as water law cases).
A small number of cases may have been decided on the merits without a
written opinion, and in a small number of cases, more than one
petition for certiorari was resolved in a single opinion. Of course,
some Colorado Supreme court cases affirm Court of Appeals rulings.
Thus, about a third of Colorado Court of Appeals cases are appealed
to the Colorado Supreme Court, but the Colorado Supreme Court actually
considers on the merits only about 3% of Colorado Court of Appeals
cases, and probably reverses no more than 2% of those decisions.
The percentage of Colorado Supreme Court cases considered on the
merits by the U.S. Supreme Court is on the order of 1%. Fewer than one
in five thousand cases filed in the Colorado Court of Appeals will
ultimately be reversed in the U.S. Supreme Court.
Other studies have shown that there are "panel effects" in appellate courts that cause appellate panels with a mix of partisan affiliations or ideologies to make more moderate decisions than appellate panels with all judges on the panel from one faction.
Partisanship is particularly weaker below the U.S. Supreme Court because all other courts are bound to follow the precedents set by higher courts.