We subscribe to OMNILERT's e2Campus service, but have never used it outside of testing. The only concern we've had is managing the registrations. People sign up for the service, but at some point the registration automatically expires (one or two years, I don't remember right now) and then they get a cryptic text. Every time this happens we get a spike in help desk calls. We also don't have a good way to have people remove or alter their registration when they leave or go on internship.
We are moving forward with Rave Alert mobile saftey. In querying a couple of other places, we found the three main ones to be e2Campus, Rave, and Regroup. They all seemed to be about the same, but we liked some of Rave's opt-out options compared to opt-in, which we find will be better for our situation.