While having random ads inserted into their messages might not work for some developers, having access to a free SMS gateway is a very seductive proposition, especially for small developers who don't have the resources to either establish relationships with the big mobile carriers, or who simply don't have the money to pay a commercial provider for access to their SMS gateway.
We asked Zeep Mobile if it had any plans to share revenue from these SMS ads with developers or if it was going to give developers any influence over which ads it would display. However, as of now, Zeep Mobile is not planning anything in this direction, though it would seem reasonable to assume that they might start sharing revenue with developers at some point in the future.
Besides the standard web API, Zeep Mobile has also developed Python and Ruby libraries, which developers can use free of charge. As far as we can see, this API is pretty straightforward and well documented.
Still in Beta
It is important to note, though, that the service is still in beta right now and that developers are limited to sending out messages to no more than 10 recipients at a time. Also, while Zeep Mobile has plans to expand beyond the U.S. in the future, the service can only send messages within the U.S. for now and is also limited to a select number of carriers.
Overall, though, this seems like a service worth looking into for developers who wants to start adding some SMS functionality to their applications without having to pay some of Zeep Mobile's competitors like SMS Everywhere, Clickatell, or Celltrust.