I also have used it, I also tried Alien Skin Blowup and they both do very good job, the only thing I liked more about Blowup is that its faster to process, Genuine Fractals takes for ever.
I used both blowing up a 5x7 to 36x54 and they both look very good no big noticeable difference between them, just the speed.
BlowUp time : 2 Min. 10 sec.
Genuine Fractals PrintPro 5.0 time: 7 Min. 47 sec.
Maybe thats why BlowUp costs about $60 US Dlls more than G Fractals
