  Quantum Hall states at filling fraction $\nu$=5/2 are examined by numerical
diagonalization. Spin-polarized and -unpolarized states of systems with $N\le
18$ electrons are studied, neglecting effects of Landau level mixing. We find
that the ground state is spin polarized. It is incompressible and has a large
overlap with paired states like the Pfaffian. For a given sample, the energy
gap is about 11 times smaller than at $\nu$=1/3. Evidence is presented of phase
transitions to compressible states, driven by the interaction strength at short
distance. A reinterpretation of experiments is suggested.
