99% of the time, SMS messages arrive within 30 seconds of the sender hitting "Send". When you are going to be delayed by 5+ minutes, 30 seconds rounds down to "instant".
As for the inconvenient, I personally HATE getting a call when I am either in a very quiet business lobby, or very loud social event for someone to deliver an essentially 1-sided trivial message ("I am running 15 minutes late"). SMS's are PERFECT for this. They are, IMO, cheap quick and convenient.
Perhaps it is all in the eye of the beholder.
The "killer app" for this situation would be (IMO) to use a GPS-enabled phone with some rudimentary mapping capabilities that would auto-text SMS ETA messages once you are >5 minutes away from the meeting start time and your travel time is also >7 minutes (ie: when it predicts that you're going to be late by more than a couple of minutes). You would have an "app" on your phone that sent SMS messages to your co-attendees (they would NOT have to install, download or use any sort of application just to get this trivial text info). This would increase the viability and adoption rate, as suddenly your friends do not all need smartphones to get some benefit from the app.