I like Gigapixel very much. I do a lot of photography with a high res camera, and like to bring out the most detail out of the picture and print it big.
Also, sometimes combine with Topaz Studio, specially AI clear which cleans up a lot of noise.
I just used it on a low res file they sent me to blow up for a 5'x10' sign... well, the original was little but not as bad as some, so it did a decent job.
Like Fred Weiss says, the software will do its own magic, but result is usually darn good.
I also have ON1 Resize, but am preferring Gigapixel, and both of these far ahead of trying to resample in Photoshop.