You definitely want to send something out that's as memorable as your wedding will be.

1) Send them out 6 months before the ceremony. Any earlier and you run the risk of your date or venue changing.
2) If you want to save money, only send them to out-of-town guests and people you know will need to plan far in advance.
3) They are a must for destination weddings, or if your wedding happens to fall on a 3-day holiday weekend.
4) If you're going with a quirky photo or motif, be cautious about who you're sending them to. Especially if you have any relatives on the conservative side.
5) Be upfront about who's invited. Does the guest receive a +1? Are you banning children from your wedding? It's important to note this in the content of the card.
6) Remember to add "Invitation to Follow."
7) Don't forget to include your wedding website URL.