Already played three runs on it, and I loved every single one so far. This gives a level of variation to the game which has never been used before, and it's awesome.
Even when you do more plays, the different classes make you consider different techniques to try so that it's never just the same thing all the time, and that kind of variation in the gameplay is a lot of fun.
A lot of the monsters are tougher too. But not in a bad way, and it really considers you to try your spells a lot of times instead of always just the weapons. You will need to use some extra keys, but it's real easy and quick to learn.
Give this a play. You won't be disappointed.
Marathon: Istoria is a single-player RPG scenario set in the Marathon universe and designed for the Aleph One engine. It pushes the engine to its limits by using advanced character progression, combat, and storytelling.
Customize your character by choosing from seven different player classes, each with their own unique active and passive abilities. Decide which conventional weapons you want to specialize in, and make use of a brand new arsenal of spells. Explore Istoria while discovering its secrets and how you fit in the world through a combination of terminals and communicating with the recently deceased. Istoria features an original soundtrack by Jon Irons.
NOTE: the "microphone" key in Aleph One has recently been changed to the "aux trigger" key, which is important to know for Istoria.