Say whatever you want, but it's one of my favorites!
Ok, now my point of view: we can even start from the title that was changed from "Gorky-17" to odd "Odium", i'm pretty sure it've changed a bit the understanding of the main story, because in the original version "Gorky-17" was the city where strange experiments were tested on people using teleportation devices, after which people started to mutate and "Gorky-17" became sealed off, hazardous and dangerous zone.
Thus the specop squad was sent there to investigate the matter. They've met lots of enemies on their way, each with own method of fighting. Also they found followers and survivors. I'll try not to spoil the interest here, won't get any deeper into the story. Better say that you'll encounter dozen of bosses, each appears with a cut-scene for the higher tension IMO :) Many guns to try out, some of them are really sci-fi like laser, plasma, and remotely controlled satellite with 3 different attacks with enormous area of effect. You'll venture in a ruined city, sewers, indoors with a beautifully designed surroundings, collecting various things and looking for clues, just like in a quest game.
'Bout battle system a bit: there are no enemies you can see whilst exploring the map, combat is being triggered in designated places. When that happens, load screen appears (bit annoying) after which you appear on a battle field like in... say "Vandal hearts" or "Final fantasy tactics". Who played those can imagine the field is a grid (40x50; 15x20 and so on..). So the attacks can only be made vertically, horizontally and sidelong except them remote special weapons.
You receive experience by killing an enemy, and can level yourself up to be a better fighter. There is no dialogue options to choose like in proper RPG game, so this one can't be called RPG as GameSpotters said.
The only minus in game is screen resolution and the knife in the inventory, that only uses free space :)