Where's the text I sent you?

Your text, SMS, or short code could be seconds away, or minutes. There's a reason why it's not called "instant messaging". 

Is this a problem?  Well, it depends on 2 things: the application and user expectations. For applications such as voting, it won't do if the short code campaign doesn't register viewer votes (think: TV show)  until after the show has ended. Or, for authentication (think: online banking) if you're sitting at your PC for several minutes waiting for the short code to arrive so that you can continue with your transaction (sign-on, payment, transfer) there's likely to be, ah, no transaction.

Keynote regularly takes measurements for various mobile transactions to get a sense of how the mobile Web is doing (by the way, you can use Keynote's services to do this yourself).  Below are 2 charts that highlights the performance of 4 aggregators over a US mobile carrier.  The first chart shows total time of delivery (seconds) while the second chart shows availability (percentage).

Aggregator A had the fastest response time and best overall performance relative to other aggregators. 

If you were about to spend hundreds of thousands of dollars on a short code campaign, you'd probably not go with Aggregator D.  So before you sign-up, do some testing and find out who is best (or at least not worst).