Matcha tea is a variety of green tea with a very high polyphenol content. Polyphenols are thought to provide the antioxidant benefits of green tea. Various types of green tea have differing amounts of certain polyphenols.
A 2003 study found that matcha tea had an extremely high EGCG content, "at least three times higher than the largest literature value for other green teas".(1) EGCG has been one of the most researched polyphenols in green tea, with some studies suggesting weight loss properties, anti-cancer effects, and possible improvements in cholesterol (2-6).
While these studies are far from conclusive and most have been conducted in vitro or only in animals, it's certainly suggestive that EGCG may play a beneficial physiological role. Available for less than 5 cents a bag and with no evidence to suggest negative effects from modest consumption, substituting some of your daily coffee or black tea with green tea seems reasonable.
References:
(1)http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&list_uids=14518774&dopt=Citation
(2)http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11259102&dopt=Citation
(3) http://www.pnas.org/cgi/content/abstract/99/19/12455
(4) http://www.blackwell-synergy.com/doi/abs/10.1046/j.1365-2613.2001.00205.x
(5) http://www.blackwell-synergy.com/doi/abs/10.1046/j.1365-2613.2001.00205.x
(6) http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T8P-4924JF6-5&_user=10&_coverDate=06%2F30%2F2003&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=18163346c8878921b440b872c9a328cc