Using Structured ASIC allows products to be introduced quickly to market, to have lower cost and to be designed with ease.
In a FPGA, interconnects and logic blocks are programmable after fabrication, offering high flexibility of design and ease of debugging in prototyping.
However, the capability of FPGAs to implement large circuits is limited, in both size and speed, due to complexity in programmable routing, and significant space occupied by programming elements, e.g. SRAMs, MUXes.
Every SRAM configuration bit can be replaced by a choice of putting a via or not between metal contacts.
Altera's Hardcopy-II, eASIC's Nextreme are examples of commercial structured ASICs.