The pros are what you can imagine: immediate access to a convenient way to get lots of gain.
The biggest con with the incline trainers is that as the grade increases so does the real speed relative to the displayed speed. My x11i adds about 16% at grades of ~25%. (So if 5 kmh is displayed, the treadmill will actually be running at 5.8 kmh.) Scott J.’s incline trainer runs about +12%.
This is less of an issue if you’re not targeting specific speeds or climb rates. When trying to be specific, it’ll take some calibration and conversion calculations to figure out exactly what speed you need to set the treadmill at to get the desired speed.