I use it and find it very good for large scale upsizing. I got it
because Alamy stock images require 40meg tiffs and suggest it should
be used for upsizing for submission.

Here are some unbiased reviews.

http://www.luminous-landscape.com/reviews/fractals.shtml

http://www.designer-info.com/Writing/genuine_fractals.htm

But remember these are based on old version. 4.1 now doesn't require
the STN format and works with all CS2 formats.

Here's on 4.1.

http://www.photoshopsupport.com/tools/plugins/genuine-fractals.html

*********************************************************

"It looked like the sort of book described in library
catalogues as "slightly foxed", although it would be
more honest to admit that it looked as though it had
been badgered, wolved and possibly beared as well."

                     _Light Fantastic_
                      Terry Pratchett

One word, if you are after good interpolation.  QImage.

See below for some comparisons:

http://hannemyr.com/photo/interpolation.html
http://www.charm.net/~mchaney/testpics/resample.jpg
http://www.americaswonderlands.com/digital_photo_interpolation.htm
http://www.steves-digicams.com/techcorner/january_2006.html
http://general-cathexis.com/interpolation.html