The games are roughly equal in terms of length & content. Book II has slightly higher screen resolution and tweaked mechanics, but otherwise it's basically just a different adventure module, not a different game.
The games are connected and sequential, but you need no knowledge of Book I to get everything there is to get from Book II. Moreover, Book II's introduction offers a cliff notes version of Book I.
I agree Book II is slightly better, but they're sufficiently similar in every respect that if you like one, you'll like the other too.
At this point I've discussed both games with quite a few people, so I'll risk a potentially strawman'y way of recommending them:
The games measure time in squares moved, just like Rogue-likes. For most people, even people who (claim to) play Rogue-likes, this is a massive pacing problem that totally kills the games. For a few, though - including yours truly - it's a joy.
The character building system is slightly more complex than Spiderweb's games, similar to and somewhere in-between 1st edition D&D and 1st edition AD&D. And like those systems, your character will be a useless spass if you don't specialise to a much, much greater extent than you'll be used to in CRPGs. To some this results in a lot of false starts and impossibly difficult obstacles. Others find it immensely satisfying to build whatever kind of specialist they want to play.
Beyond the vastly superior art direction, the presentation is very Spiderweb'y, with suitably silly opponents, great dialogue and even event/location descriptions. Just don't expect the Eschalons to be as massive as a Spiderweb game.
If those three things sound appealing, you'll likely want to play both games (really, they're not that long). If not, steer clear. Most people don't like them at all.
If you need more info still, Basilisk's forums isn't a bad place to start. Neither are the Eschalon threads at Qt3 (which I've contributed a bunch to).
Excidium said:
Aren't the devs working on a sci-fi RPG now? Any news on it?
Last I checked, Book III was coming first. But yes, he's got plans for a SF CRPG after that.