On the good side, once you are used to the keys, the text editing part is pretty efficient. Also, you have access to a lot of the other modes once you want to step out of the C++ tasks, to look at other files (makefiles, log files), keeping notes with org-mode, opening ftp windows, pretty-printing,.... Plus there's less behind the scene magic than in IDEs and you have access to everything to see where something breaks. It's not a black box the same way an IDE is.

On the bad side, you'll have to spend time tweaking packages and so on to get IDE-similar goodies like autocomplete. I can't remember if there's a similar thing as a background compilation that highlight errors as you type. But the thing with emacs is that if you wonder about a plugin, there'a good chance it exists already.