Another thing the Russian involvement means is that Moscow, to limit the extent and duration of its own costs, has that much more of a stake in stabilizing Syria and in tamping down the conflict sooner rather than later. A further implication is that greater Russian support for the Assad regime may yield greater Russian leverage over that regime with regard to any moves toward peace.
An overall conclusion is that the Russian moves mark an appropriate occasion for U.S. policy toward Syria to pivot away from feckless attempts to engineer a particular military outcome on the ground and toward greater emphasis on multilateral diplomacy aimed at finding a political resolution of the conflict. Russia will necessarily be heavily involved in any such effort. Talks between U.S. and Russian defense ministers for purposes of military deconfliction on the ground are fine, but talks between the foreign ministers will be even more important. Turkey, Saudi Arabia, and Iran also will necessarily be fully involved. None of this implies that the prospects for a political resolution of this war any time soon, even apart from the ISIS problem, are very bright; they aren't. The apparent intractability of some of the positions taken by rebel groups, even as they accept in principle a political solution, are discouraging. But exploring every opportunity for diminishing the current fire in Syria is more likely to ameliorate the problems this conflict has caused than will adding more fuel to the fire.