Everybody in these days relies on multiple web services: Gmail, Google Docs, Evernote, the stock iOS or Android apps, and all these pretty devices connected in the Internet of Things ... What IFTTT does is linking them together, through a system of applets made by the user in a very simple way (no need for programming, it's really, really intuitive). As an example, IFTTT could automatically send a completed iOS reminder to a Google Calendar, to keep track of its completion, or turn off the home lights when your phone is at a given distance form home... The possibilities are endless.
The free version has a certain lag in some cases, and one should beware about critical tasks since they could for some reason fail. But in general the system is robust and does its tasks well. Sometimes you may wish some other service. They are added over time, try to pull a request.
If you subscribed to several web services, use IFTTT to connect them all and reach a whole new level of integration and synergy.
As everybody, we use multiple web services, and connecting them is extremely useful. As an example we used IFTTT to keep track of the wether during a year, just telling the system to log the data in a google sheet.