The most notable restriction is that only the Primary account holder is allowed to make purchases against the Family Pack's billing account. This makes sense for parents looking to regulate what their kids purchase, but it's not terribly convenient for say, your dorm buddies. Thankfully, MS Point cards can be redeemed by any account under the family plan, so it's possible to work around the restriction.
Also, all four accounts must reside on the same Xbox before being transferred to a Family Pack. Once an account is part of the Family Pack, however, users are free to recover them on another console. Finally, all content will be licensed to the console on which it was purchased, as well as to the account that purchased it. In other words, if little Jimmy buys an XBLA game, everyone on that console can play it, but Jimmy can play it on any console so long as he logs into Xbox Live.
$99 is definitely a good deal for four Live accounts, but we suggest reading through the Family Pack FAQ before taking the plunge.