Is it permissible to say "Wa Ni'ma Billah (And how excellent is Allah)" when reminding someone about Allah?

The saying "And Allah is indeed excellent (Wa Ni'ma Billah)" is a good expression, and there is no harm in it. Rather, it is a praise for the remembrance of Allah or an expression of wonder at His excellence, and that He is the best to be remembered. A similar style has been mentioned in the Holy Quran in the verse: (Sufficient for us is Allah, and [He is] the best Disposer of affairs) [Quran 3:173], which is a praise of Allah and that He is the best upon whom a servant can rely.