I got an inline for my new tank, based on recommendations from some people who had them, and the fact it's one less thing to hang in the tank.
I also like the fact that it has an electronic temperature sensor, which drives a seperate internal relay that turns power to the heating element on and off.
You read alot about heater failures, either stuck on or stuck off. Given how most of them are designed, it's not surprising. The problem is that it is real easy to design and build a cheap heater that mostly-sorta works, so that's how most of them are built - what do you want for 10-15 bucks? They use a simple bimetal switch to both sense the temperature, and switch the power on and off. But since the thing that senses temperature also experiences the arcing and on/off transients from switching a couple of hundred watts of power, it doesn't surprise me that so many of them seem to go bad in short order. I also wonder if the recent trend to mount them horizontally has some effect on reliability. The manufacturer may not have done much testing on them with the contacts operating in the horizontal configuration.