I got my hands on an FPGA just couple weeks ago. No experience before then. Mostly intellectual curiosity, which brought me to BTC too.
Anyway the description that made the most sense to me was to think of an FPGA as a processor with no instruction set, you design that and program the FPGA with your design. Where as a CPU, for example AMD Athlon, comes with an instruction set that's immutable/unchangeable.
Here's what I have gleaned regarding resale value of FPGAs, if the device has been programmed with an encrypted bitstream then only the original programmer can alter the design and so the FPGA has that limitation. BFL falls into this category from what I've read but not the other FPGA miners.
As for uses, they are limited by the device capabilities, as was said, and are practically infinite.