The Sustainiac is lovely. A lot of people compare it to the Ebow, which in my view is unfair on both bits of kit. The Sustainiac is basically like being stood in front of a huge cranked amp, you get endless sustain (especially on the lower strings). Flip the mode to Harmonic and you get that screaming feedback sound - but it's very controllable. The really nice things about it are that it feels and sounds so natural because it's working directly on the strings, that it works clean and that it works at any volume level. There is a tonal difference when you kick the harmonic mode in - the normal mode doesn't affect the sound at all, other than pushing the output up a little, but the harmonic mode really pushes the harmonics (as you'd expect), even when you don't let it start to scream. It makes for an almost incandescent sound, really fiery. My RG is also set up to allow me to use any of the pickups with the Sustainiac, not just the bridge one.
I don't know how easy it would be to justify, really, but I had spare cash and had always wanted to try it. Much better than the Fernandes Sustainer in every respect.
The Lil'59 is very nice sounding. Compared to the stock (a V1 on my RG) it's clearer and smoother, but has a similar overall balance of sound. It's a smidge hotter too, but not so much as would cause a problem. I'd say it's a definite improvement, but not a horrible shift in the sound. You'll get a similar overall sound, but just generally "better".