No documentation is required for withdrawal (unless the IRS audits you some day). You can simply transfer the $$ to your checking account when you need to pay tuition or books or whatever.
You can freely transfer $$ between different children's accounts.
You can now use them for K-12 expenses as well as college.
There are some rules about allowable expenses, but most things are covered if they're education-related - tuition, books, computers, software, housing (in some cases), etc.

The one downside I saw is there aren't as many investment options as a standard brokerage account.