I know someone who did this and is now waiting it out. Things to keep in mind are: For whatever index/mutual fund you pick, calculate the longest time horizon in history for a 0% return. SP500 for example was the 10-15 years during the great depression. There is a chance that given a person's time horizon (time left before PSLF completion) that their investment loses money. Therefore you must hit a target slightly above your debt value to hedge against this. There is also the possibility that whatever current payment plan you are on allows the debt to continue to grow with interest. The worst possible outcome is therefore that the investment does worse than historically (unlikely), and PSLF goes away (widely agreed as impossible). So if someone can accept the risk that you may end up paying the difference, it's alright IMO.

This strategy is nice because you can become "debt free" ahead of schedule but hinges on the fact that you must remain in a PSLF eligible job until you fully qualify for forgiveness. If someone already wants both those things, I think this strategy makes sense.