It's a great series if you like the gameplay, and AC6 easily has the most refined gameplay of the series.
People who are new (espessally those who are used to FPSs) seem to have trouble getting into it, but as far as its style of game goes its unquestionably top of the heap.
The gameplay runs a sort of middle ground between arcade and realism which will probably annoy some people, but makes perfect sense on the console and it's pretty much my favorate PvP game. Even 2v2 games are awsome while you dodge missles trying to get a clean shot off at the zippy zoomy hostiles.
The actual campain is weak, for the series, but still highly satisfying, with multiple objectives in each mission that require multiple playthroughs for the perfectionist. More so considering the new 'ACE OF ACES' DLC that completely changes the mission layouts.
Really all I can do, ultimately, is recomend the demo and say 'if you enjoy that, you'll cirtainly enjoy the full game', but if you only care about the single player then i'd recomend getting it cheap.
EDIT: Yeah, space sims are kind of dead. There is one for the 360 by Square, 'Project Sylpheed' but it's not very good.