It's notoriously hard to judge the political consequences of candidate debates. The media and political elites tend to opine as either drama critics judging performance art or as professors judging intellectual arguments. Doing well on one or another of these criteria can matter for a candidate. But usually not much. More often, the key political consequence of a debate doesn't follow from who had the better performance or who won the arguments. What turns out to matter more is whether a dangerous line of inquiry or attack was opened up against a candidate, or whether a candidate succeeded in closing one off, or whether a promising line of argument was discovered.
In that respect, I didn't see that Ron Paul and Rick Santorum suffered any great harm, or discovered any great opportunities, Thursday night. I thought Newt Gingrich probably did as well as he could to try to shut down the Marianne Gingrich line of questioning for the future and convince voters to put it out of their minds (we'll see if he succeeded). He seemed to me to fend off other far less threatening jabs on various fronts pretty easily.
Mitt Romney had, I thought, a potentially problematic debate, because he failed to shut down or even to blunt two distinct lines of attack that are problematic for him: his taxes, and Romneycare. Romney left himself open to future debate, and future attacks, on both fronts, by not providing a convincing reason as to why April rather than now is the right time for him to release his taxes, and by standing by Romneycare without explaining how it's fundamentally different from Obamacare (if it is).
Neither of these may matter much—it's not as if Republican primary voters were unaware of either issue before the debate, or that there's an obvious way for Romney to have made them go away. But I do wonder whether, if you were an undecided voter, your two main takeaways from the debate wouldn't be that Romney seems evasive on taxes and unconvincing on Romneycare.