i think Genuine Fractals is excellent quality for the price -- and i havent even used it that much neither!!!

you can crop areas and then increase the size and even convert it from 72dpi to 300+ without that much loss of quality

i have found that if you have a good photograph it will do a great job 

-- if you use it on computer graphics - i.e.  a Cinema 4D render/Terragen render it creates a sort of painted look to the image