This plug is great, and for a plug as popular as this, it's hard to believe there are no reviews yet.
So what's it do?
Given almost any sound, it will bring a little unobtrusive life with low delay settings. Given a good sound, it will bring out stronger textures. Higher settings can start to change the sound into something new and more pad-ish and can easily be done well, especially if it's the first effect in a chain. As with any delay-based tool, extremely fast transients won't always sound good on high settings, but it's easy to find low settings that work.
Who would use it?
I would recommend this effect to anybody using plugin instruments or any other audio that needs a little extra life. It's free and couldn't possibly take more than a few minutes to try out. Also, notice that it has a place in the credits for Spectrasonics Atmosphere, a sample-based synth with perfectly rich, balanced sound.
Some things I do:
For pads, I'll sometimes taken the sounds from Atmosphere or any other high-quality long pad sounds that I want to ambient-ify, filter the high end a bit, run them through an instance of Hexaline, put a little feedback delay after it, and slight reverb at the end for a nice, swirling pad (and this whole effect line could be free plugins). For leads, I'll take the output from a lead synth that needs a little bite, add Hexaline, adjusting to fit, then go back and change the original synth to take advantage of the added layer of sound. I don't really use it for bass, though I'm sure if you're looking for thick sound down there, it will do nicely. I wouldn't put it on large groups of sound, but would keep it to one or two instruments at a time so it doesn't add the same thing to all instruments. Also, I don't typically use it on recorded audio, but that tends to be because I seek to preserve the essence of whatever I recorded. I use it frequently on synths. I stress that it doesn't seem to cause the sound to have noticeable strong or weak notes like some delay tools can and whatever texture is added to the sound tends to be uniformly nice.
It's free, takes only a second to try, is extremely useable, and has negligibly low cpu usage (with no stability problems [Cubase]). It brings out a little or a lot of life, as desired, and is very easy to control. I highly recommend it.