  We discuss up-to-date constraints on the Majorana neutrino mass
$m_{\beta\beta}$ from neutrinoless double beta decay ($0\nu\beta\beta$)
searches in experiments using different isotopes: KamLAND-Zen and EXO
($^{136}$Xe), GERDA and MAJORANA ($^{76}$Ge) and CUORE ($^{130}$Te). Best fits
and upper bounds on $m_{\beta\beta}$ are explored in the general landscape of
nuclear matrix elements (NME), as well as for specific NME values obtained in
representative nuclear models. By approximating the likelihood of
$0\nu\beta\beta$ signals through quadratic forms, the analysis of separate and
combined isotope data becomes exceedingly simple, and allows to clarify various
aspects of multi-isotope data combinations. In particular, we analyze the
relative impact of different data in setting upper bounds on $m_{\beta\beta}$,
as well as the conditions leading to nonzero $m_{\beta\beta}$ at best fit, for
variable values of the NMEs. Detailed results on $m_{\beta\beta}$ from various
combinations of data are reported in graphical and numerical form. Implications
for future $0\nu\beta\beta$ data analyses and NME calculations are briefly
discussed.
