The length of the impulse response of an N-th order IIR filter is N+1. 

In contrast to symmetric FIR filters, the phase response is linear in the passband. 

IIR filters are always stable. 

In contrast to symmetric FIR filters, the phase response is non-linear in the passband. 

IIR filters can become unstable. 

The impulse response of an IIR filter equals the coefficients of the difference equation.