  We present new imaging and spectroscopic observations of six ultra-compact
dwarf (UCD) galaxies in the Virgo Cluster, along with re-analysed data for five
Fornax Cluster UCDs. These are the most luminous UCDs: -14<Mv<-12 mag. Our HST
imaging shows that most of the UCDs have shallow or steep cusps in their cores;
only one UCD has a flat ``King'' core. None of the UCDs show tidal cutoffs down
to our limiting surface brightness. Spectroscopic analysis shows that Virgo
UCDs are older than 8 Gyr and have metallicities in the range
[Z/H]=-1.35...+0.35 dex. Five Virgo UCDs have super-solar alpha/Fe abundance
ratios typical of old stellar populations found in globular clusters and
elliptical galaxies. Virgo UCDs have structural and dynamical properties
similar to Fornax UCDs. The Virgo and Fornax UCDs all have masses ~2-9x10^7
Msun and mass-to-light ratios ~3-5 Msun/Lsun,v. The dynamical M/L values for
Virgo UCDs are consistent with SSP model predictions: Virgo UCDs do not require
dark matter to explain their mass-to-light ratios. We conclude that the
internal properties of Virgo UCDs are consistent with them being the
high-mass/high-luminosity extreme of known globular cluster populations. We
refrain from any firm conclusions on Fornax UCD origins until accurate age,
metallicity and alpha-abundance estimates are obtained for them. Some of our
results, notably the fundamental plane projections are consistent with the
formation of UCDs by the simple removal of the halo from the nuclei of
nucleated dwarf galaxies. However the ages, metallicities and abundances for
Virgo UCDs are not consistent with this simple stripping model. It might be
consistent with more sophisticated models of the stripping process that include
the effects of gas removal on the chemical evolution of the nuclei.
