FPGA is clearly more _practical_ if you're trying to bang out some commercial functionality. Still, making your own chip fab is cool to do in it's own right. :)
By themselves, no. But FPGAs are widely used in mixed-signal systems with an ADC on one side and a DAC on the other.
Yes, for sure. FPGAs are a great way to move around and crunch large amounts of digitized data. But there are other times where that doesn't help much because you're doing something like a submillimeter analog front end or something.