I have the actual paperwork at work, but it's something along the lines of UPT ADSC complete and less than 15 years of service. So you have about a 3 year window to be eligible. Also, you are non-eligible if you are a permanent AGR. You have to be a part timer (but can take TAGR/ADOS/seasoning orders) to be eligible. I think there was some language about if you pick up a PAGR position later, you aren't eligible for any more payments, but I don't think that will matter as it's one lump sum payment. Don't quote me on any of this.
I just signed mine today... as for tax free, the legaleze reading seemed to be that the IRS only allows (re)enlistments when signed downrange to be tax free. Apparently Officer accessions bonuses don't seem to count as an enlistment. Oh well, i've fought the system for almost 14 years... I've had enough.