I used to use them as my main tanks - now I use the 1.75g (medium) for the fish that I am conditioning to breed, and I use the .75g (small) to house the fish that I am breeding. (Went to larger, divided tanks for my pet bettas as it's much easier on me to maintain those then having to do all those water changes on all those keepers.. would be way too much work lol)
The mediums and up are easy to heat - I keep 25w adjustable heaters in there with no troubles of keeping the water as warm as I want it, nor is there any weakness to the plastic. My smalls are kept in large, shallow plastic tub filled with water and a heater in the tub to keep the tanks warm.
Yes, they can get that ugly scum on there easily, but it can easily be cleaned off with some scrubbing.
You can decorate them real pretty, just have to use your imagination!
All in all, if you can't afford a tank or don't have room for a larger tank then a kritter keeper is a great way to go. I give it an 8/10 for being a good, cheap tank alternative :)