Quintessential Difficulties
  An alternative to a cosmological constant is quintessence, defined as a slowly-varying scalar field potential V(\phi). If quintessence is observationally significant, an epoch of inflation is beginning at the present epoch, with \phi the slowly-rolling inflaton field. In contrast with ordinary inflation, quintessence seems to require extreme fine tuning of the potential V(\phi). The degree of fine-tuning is quantified in various cases. 
Christopher Kolda and David H. Lyth