Okay, played the expansion a fair bit now. Really liking it so far (well aside from some launch shenanigans).
Haven't touched the campaigns yet, dunno if I will. Not really a big draw for me, so if your main thing is campaigns, might want to skip this post as it may not be helpful.
New additions are awesome. Naval combat has been massively improved as a result of Fire and Demo ships being given Feudal Age variants, and having one tech that upgrades all three ship types to their Castle age versions (still separate techs for Imp age, though). Huge, huge difference IMHO. Now you'll actually see diverse and interesting naval battles rather than just Galley spam. Fire Ships are maybe still a tad on the weak side but at least they're no longer fighting an uphill battle due to coming an era later than Galleys (the unit they're ostensibly a hard counter to) and as a result being hugely outnumbered.
Arson tech is also a much-needed addition. I actually find myself sometimes using Swordsman line now, and Infantry civs are more viable. For those unfamiliar, it's a Castle-age tech from the Barracks that gives all Infantry (Swords line, Pike line, plus whatever unique units - except (I think) Eagles) bonus damage vs buildings. Considering how underwhelming infantry were in this game until now (rarely seen aside from Pikes), I've been a big fan of this change so far. Infantry civs like the Goths and the Japanese are now a lot more interesting.
They added a new unit, the Siege Tower, designed to make offense a bit more viable in the Castle age, giving players another option than teching right to Imperial to get the good siege. Honestly, haven't played around with it much.
A lot of general rebalances, too, that are nice. Particularly reworking Scorpion mechanics a bit to make them more useful, and finally categorizing Camels as something other than a ship. Axemen are still shite but what're you gonna do?
I haven't played around with the new civs too much yet. The new art style doesn't stand out too much, and they seem reasonably balanced. But like I said, haven't messed around with them too much yet.
In any case, IMHO the strength of the expansion pack is more in how it fixes and revitalizes core gameplay than in new civs or campaigns. Whether that's something worth spending money on is probably going to come down to what you enjoy about AoE.