Is a gas mask a good buy?

In my opinion NO. First, surplus masks and filters are sold as surplus for a reason, the military that used them updated their inventories. Second, the surplus filters are usually for training or tear gas. Third, any brand new mask and filters are expensive and that money could be spent on food, medical etc. Lastly, without detection equipment most CBRN threats will go undetected until its to late, symptoms equals to late.   

However a gas mask is good if the scenario you find yourself in is a world with excessive law for this I would suggest getting a Finnish gas mask and filter adapter or the Russian GP-7V. Do not use surplus filters buy either North, Mestrel, 3M or Scott filters.