Episode 1 - pretty slow and a bit easy, mechs in battle are almost always useless, objectively mediocre I guess but I liked it overall.
Episode 2 - Kevadu put it well, it's very experimental (and difficult) like a Saga game, shit ton of sidequests, hated it the first time but replayed it as part of a marathon and thought it was great, but the voice acting and graphics are weird, also no way to buy stuff so items are rarer than in most games.
Episode 3 - the most intuitive of the games, like Hexyz Force it's super traditional but does it really damn well with a few extra mechanics, a really cool but balls hard minigame, solid difficulty, a fantastic game imo.
Xenogears - has a lot of secondary characters and subplots that get underdeveloped, chaining deathblows is a lot of fun to watch, overall a good game especially with the way shit gets real at certain parts, has a GREAT retelling/parody/thing.
If you like the stories in Tales games, you'll probably like these stories.
Xenosaga hypothetically fits into the backstory of Xenogears via the Perfect Works chronology.
@Tiamat boosts work by characters attacking to fill up a gauge, each time it fills up that character gets a chance to cut in front of an enemy.