ETFs generally provide easy diversification with low expense ratios.  They also have the tax efficiency of index funds , while still maintaining all the features of regular stock.   ETFs can be acquired economically, held and then disposed so some investors invest in ETF shares as

• a long-term investment, or
• a way to implement market timing strategies.

ETFs Advantages:

• Generally have lower marketing, accounting expenses & other fees
• Flexible in purchasing times (ie. any time of day)
• Tax effective
• Market exposure – introduce your portfolio to a variety of markets
• Usually lower than other financial investment products

I think i managed to list all the advantages.  If not, please post your comments and let me know what you think.