If you have yearned for more dynamic diplomacy and the possibilities for world war, this is the expansion pack for you.
Balanced map, standard size (or larger if you are into big games that take days to complete), aggressive AIs, and --this is important-- at least 2 or 3 civs more than the minimum for the map size. On standard, try 9 or 10 civs, for instance. Larger maps could take more, of course. I play on Prince level.
What this does is setup the right conditions for the new vassal system to really engage. If not in every game, certainly in most games under these conditions you will find yourself in some truly epic battles with all kinds of change of fortune. You'll be alternatively loving and hating other civs as you all eventually become tangled in a great global mess where Master and Vassal(s) develop a kind of inertia that is far greater than the sum of their parts.
And vassals aren't just an easy way to mop up the end game (as Gamespot suggests). For instance, you can tell your vassal what to research, which opens the strategic possibility of having your own customized tech trading partner. Vassals will also help you in wars. But both of these suggest that your vassal is healthy enough to be of any use. So do you gift your vassal a lot of tech so it can actually research something you need? Do you build up the vassal in other ways so that if war comes, it can actually help you? Then again, if the vassal breaks from you (because you aren't protecting it well enough), you could have just raised a bitter enemy bent on total revenge. Again, you need enough civs close enough to each other for all this to really have significant meaning early enough in the game...or, yeah, a few end game vassals might seem like little more than a way to save yourself a headache (which is great, too!).
To me, this represents a fundamental move forward in how the war aspects of Civ behave...and it's long overdue!
Of course, if you don't like this kind of cut-throat game (maybe you enjoy peaceful builder games with relatively empty or isolated maps so you can grow confidently), then the vassal system might not often kick in to the degree I'm describing. At that point, ask yourself if you enjoy interesting scenarios? If so, Warlords is full of them. Also ask if you just want more quality content (new civs, traits, great generals, wonders, unique buildings, etc.). If so, Warlords is a safe bet.
Overall, if you've been enjoying Civ 4, Warlords is almost certain to add to your enjoyment. Whether or not you'll find it worth $30 depends on what you are expecting to find, and hopefully my and other reviews will help.
--Yin