http://www.japanparts.com
the only negative is cost, and a little weight. sometimes if the bar isnt a well designed unti you can have clearance issues, or fitment problems(IE hitting the valve cover, or holes not lining up right), but I dont think theres any real "con" to them other than those...
get a good quality unit, like the NISMO, cusco, apex-i, or other good brand(stillen makes a nice one too). and you'll avoid a lot of the problems I listed.hope that helps!-chet