I have used XST a couple of times and my experience with it was really bad (It comes with ISE Foundation and is not X!linx main concern, nor their main income 8O , they focuss in their PAR tools).
Synplify Pro, I like it! Latest version I have tried 7.3.3 is pretty good, is synthesizing designs it wasn't in version 7.2.3. But there is a PROBLEM, at least is a problem for me, you have to write your RTL with Synplify attributes in many cases, so your HDL code becomes Synthesizer specific and that is scary.
Leonardo = Precision, I love it! This is my favourite. It has improved the Schematic Viewer that was lacking before compared to Synplify. It doesn't need attributes in your RTL. Historically hasn't had as much problems with RAMs as Synplify. It does an area report by entity which really helps when optimizing designs,...
-maestor