1.) It requires a separate data plan, though it's still often cheaper than having a separate home connection.

2.) It requires constant 3G access so you'll want to test your current coverage throughout your home to verify that there are no dead zones, especially towards the interior where there's potentially more interference.

3.) It requires your phone to be present, so family and guests will not have internet access if you leave for whatever reason unless you leave your phone behind.

Aside from those potential issues, it's a solid alternative.

Hope this helps,
John