It's so-so for renders but it's really useful for photography. Most renders are too pure for this software and look more unreal than before scaling. You can try it however with some vegetation-rich renders.
I've also tried Alien Skin BlowUp 2 and it looks slightly better to me compared to Genuine Fractals.
Also, it's a beast when you have some low resolution, raster logos to put for print and you don't have access to vector versions.