If it is actually criticism (as opposed to blatant, unjustified, unsubstantial attacks) I see/take it mostly as a negotiation of sorts usually after searching for the understanding, e.g. different priorities, fix of flaw, lack of possibilities, etc. Then you can simply evaluate whether the criticism makes sense.
I do think in many cases it makes sense for someone to start with questions to understand the ideas first, but that often doesn't happen. (Or provide reasoning and/or context to the criticism, to make it useful.) I found it also isn't always immediately obvious which aspects is emphasized with criticism, especially if (too) concise.
Of course, if they attack the person it's not really criticism anyways. 

 Yea.. if they simply attack to attack its best just to ignore them. 

 exactly.   

 Actually, 'negotiation' here means trading off priorities, features, ideas, etc. (As opposed to involvement of money.)