IMO it depends entirely on how you set it up, but even when it does work, it's no guarantee.
If you apply silicon on only one side of the acrylic, and then have a higher water level on the glue side, the weight of the water will eventually (if not immediately) pop the acrylic loose.
However, if you use thick enough material, and cut the acrylic to fit tight into place, and heavily glue both sides with a good 1/2" side bead across all seams, the silicon will tend to hold the acrylic in place, wedged between the beads. What I will say though is that it might not necessarily be water tight. For inter-sump baffles, it's usually no big deal, but if you have very low flow going through one section... not enough to overcome the rate of leakage and keep the section full, you will have issues.
Another method I've heard is to use Marine Goop to physically secure the acrylic to the glass, then use silicon to actually seal it. Silicon will seal to acrylic as long as it doesn't have any physical stress on it acting to pull the arylic loose.
Regardless, what Sign guy said is right, it doesn't stick, and although some people can claim success, it may have more to do with the circumstances of how they did it, their particular setup, environment, etc than anything else.
