There’s a significant amount of comment spam going around these days (although a lot of it is directed at MT, the rest of us still have to worry) so I personally check all comments before they appear on the site. Don’t be surprised if your comment doesn’t appear within 5 minutes (and especially don’t submit it again)- it will appear the next time I get a chance to go through the moderation queue (usually every 24 hours, except at weekends).