I finished for the moment the first two episodes.
The first is an HD version of the one released on PS2. Graphically, it worked pretty well aged. It's not terrible, it is not of exceptional quality, but it's still okay. The gameplay is good, life rather correct (I'd say about ten hours of memory), but I advise you to boot directly in hard mode for the challenge.
Interesting idea exploited in this episode: the choice of one of 4 unlockable characters in the country, each with a different style (the first character is standard, the 2nd most in Stealth, 3rd nag at will and 4th quite similar to the first). It does not change the game radically, but it's still nice. Note the presence of a multiplayer split screen.
The 2nd radically changes the game level graphics: this is really nice. The gameplay remains the same: effective, and it will not radically change from the first episode. The game is long (veteran actually) with passages difficult when same well (especially the palace of Visari, the last level of the game, do not eat your controller if you are :-p rabies). Note multi online mode which seems to be quite complete but I have not tried. Only regret is not playing a character 1st opus, although we see them from time to time in the cutscenes of the game.
I can not wait to try the third episode, Guerrilla Games managing quite well license (especially at the technical level, just seeing Killzone: Mercenary PSVita to be amazed).
We will return by cons in the history, which does not enthrall crowds, although the universe is looking, but a little dull in color (something that should change with Shadow Fall).