If you are familiar with verilog and digital circuit design,it's not difficult.But it is OTP FPGA for its anti-fuse tech.This is the biggest difference with Altera&Xilinx's SRAM tech FPGA.
I have use the QL4090 and some other ones. I think it is quite easy to use especially the software tools but the only problem with Quicklogic, I believe they are one time programmable. Not exactly a FPGA!! I wasted 6 of them trying to incorporate simple change. I had no other choice as they where small, ceramic, and fit in the board and cheaper compare with Xilinx, Altera at that time.