PhotoZoom is as good as they claim -- probably even better.
The starting image was about 20x25 pixels. The enlarged images are each about 450 x 550 pixels. That's a twenty to one linear enlargement and a 400 to 1 increase in file size. Here are the results.
Phil I use PhotoZoom all the time. My experience is that a good quality image can easily be enlarged 500 %. That is a 5" image at 300 DPI can be used at 25" at 300 DPI.
IF THE IMAGE IS GOOD to start with.
I regularly have 7" images at 300 DPI which is 14" at 150 DPI print at 150 DPI at 96".
That's a good max limit for quality and to get 200 or 300% with a good starting image is no big deal.