So not only can you set customized replies, you can also choose how long you're doing the activity, so the phone will only reply "In a meeting" during the two hours that you set that you're in a meeting. (Would be nice if this synced up with your Google Calendar too, to automatically know what you're doing.) What's also cool, is if someone calls you during this time, SMS Replier will auto-reply to that as well, saying you'll call them back when you're done.
Other features: Up to 5 numbers that you can add to get incoming calls and texts. Say, for emergency calls. Or also set numbers that you don't auto-reply to.