@Steve_SmithJun 19.2013 — #Strength is increase back link and get higher in search engine. Weakness is only that time when google changes there algorithms. But its harmful only that sites those use low quality links and spamming.
@web901Jun 19.2013 — #Strength and weaknesses of SEO are stretchable concepts that depend upon quality of backlinks that you create. The high quality backlinks will never harm your ranking even after Google algorithm modifications.
@kpkarthikJun 20.2013 — #The main strength of SEO is make a website's position as 1st position or 1st page in search engines.But in the field of SEO,we should workout on daily for making good result.If not work in single day,then the result would be going back.
@HanasonsJun 20.2013 — #The main strength of SEO is it gives long time result in no money, once you optimized the site you would get result for longer period of time.
Other thing is there is too much competition so you have to fight with your competitors as well.