Not bad. I've had my 1200 for a little over a year and a half. Recently had to replace the impellar which is the first time I've had a problem with it other than suction cup issues. Can't complain for $20.
I've had my 900 for just under a year without any issues.
I bought a 900 for my serra tank a couple years ago. When the little pump on my 50 gallon outdoor pond died (which only had plants in it), I wanted a quick replacement, so I moved my serra's 900 to the pond. This spring I put a sponge filter on it, added a couple goldfish, and all has gone well this summer with it as my only pond filter. The last couple weeks it's been a bit difficult to restart after cleaning, so it may need a new impeller, but the water in the pond has far more silt and crud in it than aquarium water, which probably causes more wear.
I bought a replacement 900 for my serra tank, and it's been running perfectly.