i like it a lot!
good things IMO:
-adds more options--more leaders, more traits, UBs
-vassal states can sometimes be cool, if you get a useful vassal or if you want a conquest win to go faster
-great generals rock hardcore and i miss them when i go back to vanilla
things i don't like about it:
-too many choices! i'm indecisive, i have a hard enough time deciding who to be in vanilla, that's my own personal issue tho *giggle*
-the vassal state concept is very cool but IMO they did not fully test it and it isn't implemented as well as it should be. i usually play with that option turned off. even tho i now understand the mechanics of how it is implemented, it leads to headaches and isn't the way i'd do it if i were queen of the world
-the opening movie and the game music. some folks prefer that stuff warlords a lot more than vanilla, i like vanilla. no biggie, you can change the music to whichever you like anyway
it does not run any slower for me than vanilla does. it also does not take away the option to play vanilla. they show up as two separate installs, so at any time you can play the one you're in the mood for. the only risk you're taking is the money if you don't end up liking warlords. i don't know your budget so that's a decision you'll have to make for yourself.
the great generals are really awesome. some of the UBs are way cool. the new traits are fun, but in rearranging the existing leaders and adding some new ones, they actually took away some combos that used to exist. for example, there's no creative/philo or fin/org leader in warlords, quite a few combos like that have gone away. so sometimes i do still go back to vanilla.