It will be interesting to see how the experiment works out.
The problem is that the real argument in favor of a guaranteed income program, in the form of a universal basic income, is the argument that Hayek gave, and to see that he was correct requires a UBI to be the only poverty-alleviation program, replacing all means tested programs. In brief his argument is that there is a difference between paying people whether or not they work (UBI) and paying people not to work (our current means-tested system). The latter creates a far greater incentive to try to remain on the dole since they impose what is in effect a confisctory tax rate on entry into the job-market: one effectively earns only one's take-home pay minus the benefits lost by no longer meeting the means test. Hayek argued that a UBI was the only poverty-alleviation program that does not create perverse incentives in the job market that effectively mire the "beneficiaries" in poverty.
Overlaying a means-tested system with an unconditional income doesn't fix the problem with the means-tested system. On the other hand, if they find beneficial effects, it would provide an even stronger argument for moving to a UBI.
