I find SMS text-entry slow and painful, and belive it's popularity in Europe stems in large part from the high per-minute charges Europeans endure. But taking a picture with your phone and mailing it to someone is quick and easy, so I think it will prove quite popular.
In Europe you are generally charged per second (not per minute), so
giving a quick message "via voice" might not be more expensive than
sending an SMS. The per minute charges are comparable to our charges here and sending messages is actually often more exensive there than here in the US. Sending messages is less intrusive than making a call, both for the people around you and for the recipient.
Entering a short message on a reasonably modern phone with T9 is quite fast. Even on my 3(?) year old T68i it's pretty good. I'm sure it's even better on newer phones. And besides, we did pretty well way back when when entering a "c" always meant three key presses. :-) Practice, practice.
Sending a picture serves a completely different purpose. I can't imagine how you can (quickly) send a picture to tell that you'll be 15 minute late. And you can't always call either, because the meeting might be started and noone will pick up their phone to take a call.
Getting a GSM phone here, I was surprised to find that the providers finally have figured out to let you send SMSes to phones on the other networks. Without that it was no wonder that it wasn't in common use.
And unrelated to my point above, did I tell how neatly OS X integrates SMS'es when you connect your phone over bluetooth?