ADVANTAGES:
Internet marketing, also known as SEO, is typically considered the most effective form of marketing, online or offline. That is because having Internet marketing performed correctly and ethically on one’s Website allows the site to reach the most online visitors, across the most states and countries, across many search engines, for numerous keyword terms.
Other advantages include: being able to build brand recognition, a site can achieve first page keyword rankings across search engines like Google, MSN, Yahoo and others, eliminating costly PPC programs if a site is optimized naturally/organically in engines, and a site owner can see a fast ROI (return on investment).
DISADVANTAGES:
Internet marketing, while usually a monthly ongoing maintenance fee, may still seem too expensive for businesses working out of their homes or small business trying to find an affordable way to market their websites online. In addition, once an Internet marketing or SEO professional completely stops maintaining your Internet marketing program for you (and you do not considerd yourself to be a qualified professional), the rankings a site owner worked so hard to achieve in the search engines eventually drop/fall in the results (nobody working on keeping them there anymore). Getting high keyword rankings and keeping them takes ongoing commitment and time, so when nobody is there anymore to continuously work on the rankings, they will fall. One last disadvantage I can think of is that sometimes Websites simply cannot be optimized at all or have Internet marketing performed, due to a number of reasons, such as the Website was designed/built in a way that search engines cannot find or recognize pages within the site in order to optimize it (frames, flash websites, dynamic HTML pages, etc).

See these articles:http://www.glendaledesigns.com/articles/…http://ezinearticles.com/?Advantages-Dis…http://www.extension.iastate.edu/AgDM/wh…http://www.nowsell.com/web-marketing-str…