I have been using BlueMountainArts to send e-cards for a long time now, and I have to say that it is my favorite e-card site. The selection is excellent, and I have never had any trouble using the site.
BlueMountain also has a great feature for planning parties and managing invitations. I used this feature the last time I had a party in my home, and it was absolutely wonderful to receive email notification of who had confirmed attendance at the party and who was not going to be able to come.
If you are looking to send an e-card to someone, check out BlueMountain before you go elsewhere. They also have a list of unusual holidays on the site, which makes it easy to be creative in card-sending.
One of the main reasons I like this site so much is its interface. The site moves quickly and is easy to deal with. I have not had trouble with pages faulting out or irritating lag time when using the site.
It has a preview feature, and you are also able to send the card to yourself and receive notification when the person views the card you have sent them.
BlueMountainArts is a great site; give it a try!