I've never used Blowup, but I do have Genuine Fractals 5.
I recently tried to blow up a really, really small photo, extracted from a PDF and only about 350KB in size and 200ppi, to print an A2. I found with GF5 I was getting really blocky output, very posterized, I know it was a big ask to blow such a small file. I was also getting a lot of out of gamut colours.
I wasn't happy at all with the output, and eventually I just used Photoshop's (CS3) own image resize option, with BiCubic smoother and up resampled to 300ppi. And that gave me near perfect results (as perfect as one could get with such a small file).
So, sorry I can't say I really recommend GF5, but then maybe I was asking too much of it with such a crappy file to start with, maybe with a better original it'll give better results then PS, but I haven't had a chance to try that yet.
Interested to hear what others have to say.