  We present a comprehensive space-based study of ten X-ray luminous galaxy
clusters (L_X>=8e44erg/s[0.1-2.4keV]) at z=0.2. Hubble Space Telescope
observations reveal numerous gravitationally-lensed arcs for which we present
four new spectroscopic redshifts, bringing the total to thirteen confirmed arcs
in this cluster sample. We combine the multiple-image systems with the
weakly-sheared background galaxies to model the total mass distribution in the
cluster cores (R<=500kpc). These models are complemented by high-resolution
X-ray data from Chandra and used to develop quantitative criteria to classify
the clusters as relaxed or unrelaxed. Formally, (30+/-20)% of the clusters form
a homogeneous sub-sample of relaxed clusters; the remaining (70+/-20)% are
unrelaxed and are a much more diverse population. Most of the clusters
therefore appear to be experiencing a cluster-cluster merger, or relaxing after
such an event. We also study the normalization and scatter of scaling relations
between cluster mass, luminosity and temperature. The scatter in these
relations is dominated by the unrelaxed clusters and is typically sigma~0.4.
Most notably, we detect 2-3 times more scatter in the mass-temperature relation
than theoretical simulations and models predict. The observed scatter is also
asymmetric - the unrelaxed systems are systematically 40% hotter than the
relaxed clusters at 2.5 sigma significance. This structural segregation should
be a major concern for experiments designed to constrain cosmological
parameters using galaxy clusters. Overall our results are consistent with a
scenario of cluster-cluster merger induced boosts to cluster X-ray luminosities
and temperatures. [Abridged]
