Operation Darkness is a good, enjoyable turn-based strategy game that SRPG fans shouldn't pass up. It's long, challenging, features a lot of interesting mechanics that make the experience work well, and the concept alone is insanely interesting, which is something the plot manages to run with reasonably well. It's not the prettiest game on the market (sacrificing graphical quality for troop quantity), and it's pretty difficult if you're not used to this sort of game.
Even if you are a veteran SRPG fan, the camera can be unwieldy at times, some of the difficulty mechanics might be off-putting, and some of the mechanical design choices are annoying or questionable. That aside, if you're up for a challenge and you can look past the mechanical problems, Operation Darkness is one of the better SRPG's on the market for the serious fan, and it'll keep you busy for a good, long time.