I wonder if OpenLayers 3 is trustable enough to start migrate application to it now. Is the difference between versions 2 and 3 are too much important that the old code wont works? Are the major improvements of OpenLayers 3 is tested enough like 3D?
OpenLayers 2 will stay for a while because of its history: it's the first big Open Source API for mapping. So, people invest a lot and they can't switch: there is no backward compatibility between the two versions. FYI, the 2.13 version is in 2.13 RC2, so it means the official release will land in some weeks.
OpenLayers 3 is a rewrite of OpenLayers with a new API that can handle 3D, using HTML 5 features like Canvas and WebGL. WebGL and Canvas also enable you to display more vector features than with OpenLayers 2 (you don't have DOM nodes limitation) It's up to you to check OpenLayers 3 and give feedback on the mailing list. Features are autotested with Travis CI and js unit tests https://travis-ci.org/openlayers/ol3.
OpenLayers 3 requires a recent browser because it relies on recent technologies. Don't expect IE6 support in OpenLayers 3!! At the moment, you always have more features in the OpenLayers 2.
So to switch need you to evaluate what features your current application requires in order to make your decision. Make a list of the features you are using at the moment in a spreadsheet and compare with the features available in OpenLayers 3 already, looking at the examples.
I diecided to migrate and I find it a bit difficult. Currently most of the classes are untested and many other things should be checked before using them. I'm not a pro in JS but it seems a bit more complicated than it was before.
My answer - if you are not in a hurry using Vector data and 3d just wait a little more.
not an expert, but I know that a lot of people still work with OL2 (2.10, 2.11, 2.12). It could be because upgrading is 1) time-consuming (very likely rewriting the existent code), 2) OL3 seems to be so far worldwide untested and in somehow "undebugged", 3) GeoExt2 and Ext4 also are still being "tested" (so people probably are still working with 1.1 and 3.4 respectively).