The vast majority of reports suggest that
PMMA in its final form (a good 6 weeks out post-op) is rubbery at best, but not squishy. Will you find some exceptions, sure, but that's true for everything.
Most fillers aren't really squishy when injected in modest or conservative volumes. The only truly squishy filler I know of is silicone. Not only is silicone squishy, but poses higher risks overall.