I think they're on good ground. The judge who threw out net neutrality last year, basically outlined what they would have to do legally for the rules to stick:

"Given that the Commission has chosen to classify broadband providers in a manner that exempts them from treatment as common carriers, the Communications Act expressly prohibits the commission from nonetheless regulating them as such," Judge David Tatel wrote for the court.

http://www.wsj.com/articles/SB10001424052702304049704579320500441593462

So with the ISPs being classified as "information services" (classified as such under a Bush-appointed FCC chair, now TV-industry lobbyist Micael Powell) in previous years the rules were invalid because information services are basically content providers like any other blog or news paper, and thus should be able to say (or not say) whatever they want.

This year, as you know they've been reclassified as common carriers, because none of us want ISPs to have any say in delivered content. Which would put the rules in line with the judge's expectation with the previous ruling. So I think the FCC has some weight on its side because they changed exactly what the previous judge took exception to, not to mention the millions of public comments it received. But that doesn't stop legal challenges, and the result of it might be a few loopholes added here or there in favor of ISPs, but I think the overall set of rules will remain. I think an ISP would have challenge the existence of the FCC itself to kill the rules, and I don't think that's a fight anyone could fight.

But I'm no expert. Just a lay person who's been watching.