Remains to be seen. Both airlines currently concentrate more on the east coast, so there will be some overlap in routes and bases that might lead to job losses. JetBlue has more of a presence on the west coast, but neither airline operates much in the midwest at all (no flights anywhere in the Dakotas, Oklahoma, or Wyoming, for example). Overlapping east-coast routes would lead to opportunities to employ aircraft elsewhere, however, and I think we would see moves further west to establish new bases or focus cities. JetBlue brings flights to the UK to the table, and maybe more European flights in the future as the Airbus A321LR and A321XLR jets on order get delivered. Spirit brings more of a presence for "vacation" destinations in Central America, South America, and the Caribbean to the table. Both Spirit and JetBlue operate mostly Airbus fleets, so fleet integration shouldn't be much of a problem. Both airlines have quite a few new Airbus aircraft on order, and by 2025 or so the combined airline could be something like the sixth largest airline in the world by fleet size.
So this good or nah?
I see the combined JetBlue turning into much more of a competitor for Southwest, in particular: a low-cost carrier that offers a nationwide network.