There's an interesting post at Liberal England: Why are the Lib Dems so bad at PR elections? that wonders why the Liberal Democrats seem unable to rake in the votes once they get a "fair system" [sic]. (Their latest setback has been the loss of two of their five London Assembly members. Now the Assembly group can hold its meetings on a rickshaw!)
Some of this may be down to the particular system used - see my comments on the details - but it also suggests that the Liberal Democrat vote is rather softer than many think, and only snowballs when ruthlessly targetted. When it has to compete with other options and can't hide behind misleading bar charts it becomes exposed to the winds and does weakly, sometimes even worse than under first past the post.
So where does this leave the campaign for proportional representation?