OP AMP as active negative resistors
Question to the gurus: How practical is this? Can I build a op amp circuit and replace one of the gain resistance with this "active" resistance and control the gain using voltage (like VCA)? 

No - you can`t. I suppose, you are referring to the circuit called NIC, correct? Please note that this circuit offers a GROUNDED negative INPUT resistance only. Hence, it cannot compared with a classical passive ohmic resistor. More than that - how do you intend to "control" the value of this "active" resostor? It has a fixed value!

However, regarding your first question; This circuit is very practical. It is extensively used in oscillator and filter circuits. In these applications, this negative resistance can be placed in parallel to any (unwanted) ohmice resistance - thus reducing/compensating resistive damping properties because of [R1||(-R2)]>R1. 

As another important application the NIC circuit is part of the two-opamp circuit called GIC (Generalized Impedance Converter), which is used to create an active inductance or an artificial block called FDNR (Frequency-dependent negative resistor). Both blocks are extensively used in active filters.