I am currently using Unigraphics, ProEngineer, Catia V4 and V5. In my opinion, Unigraphics is the best tool. It is the most robust, most stable and most flexible tool available. There is no other tool that can handle customer data easier than Unigraphics. As a design tool, Unigraphics is the most flexible. You can use solids, surfaces, wireframe all mixed together. You can use parametrics if you want but if it does not lend itself well to your work then you don't have to. You can use explicit modeling instead. What is most important is associativity (if a change is made, will all your other work update) Unigraphics is by far the most capable tool in this regard. It is also the easiest tool to trouble shoot if there are problems with your design.
ProEngineer is far to constricting of a tool. It is very common to design yourself into a corner using ProEngineer. This waists a lot of time and effort. ProEngineer is known for being too difficult to use. CATIA V5 is a good tool but it is far to unstable and not even close to the capability of Unigraphics. It has been developed for over four years and is still not production ready. You can ask many of the large companies using it that question. I know many companies reverting back to CATIA V4 (which is like working back in the stone ages) because CATIA V5 can not do their job. I have not used SolidWorks. I know many people who like it but I would tend to stay away from it because is does not scale at all to the growth of your company. In my honest opinion, Unigraphics is head over heals the best CAD solution available.
Best Regards,