I got Fireplace 3D Lite this summer when it was free (woohoo!). That is what I am commenting on (same company's product).
Works like it is supposed to and seems stable, although I don't run it constantly. However, it does seem to use a fair amount of CPU, as my computer (mid 2010 MPB 17" running OS 10.6.8) does heat up noticeably after running it for a bit, although the RAM usage isn't too heinous (116mb); consequently, I don't run it for extended periods.
The App description does say it warms one up, so I guess they're some truth in their advertising. This "feature" might even be helpful if you're sitting in the cold somewhere and want to keep your battery above minimum temperature.
The graphics are every bit as good as the virtual fireplace one gets on cable tv right before X-mas, so in that sense, this is a successful product. Would like to see smoke generation feature; however, I guess that will have to wait until Apple installs an olfactory card. Maybe that will be added to OS X 10.8 (Mountain Lion? Bobcat? Cheetah?).