Safe? Yes, they are. Have done 'em / seen 'em done for years, and until proven the other way, no problem.
Supposed engine wear? Well, heard that on the 320 Bus series the savings are in the order of $150 per degree of flex... good and easy way of saving dough
SOP? Yes, in most airlines flexing is part of the SOPs. TOGA might of course be used in windshear and eventually LOVIS ops.
Happy about it? Any decrease in safety due to flexing as opposed to using full thrust for TO should be small. Continuing this line of thinking one could argue about fuel planning, auto brake used on lo instead of hi, etc etc
Well, increasing safety to a max one could finally stay on the ground
live 2 fly 2 live