I'm playing Chaos and Empire with the same rate and sometimes I get the chance to playtest against them with the other two races...I've to say that it's pretty strong for its capability to control the table.
It has a very basic/annoying table control (corruption, direct damaging) and a bunch of weird effects (Shrine to Nurgle, Warpstone meteor - that I really love -)...Than, we have a good pool of Tactics to choose from (Blood for the Blood God, Flames of Tzeentch, Slaneesh's Domination).
Bloodthirster is a wonderful surprise: at the beginning, I used to see it as an almost unplayable Unit...I usually don't like big monsters like that...But, believe me, I play it every match.
With a good Kingdom Zone Building (it's the real cornerstone, IMHO) you let it hit the table at turn 4/5. Tonight a friend of mine played Chaos a couple of times and in our 2/3rd game I had to handle TWO of 'em...Dramatic.
Then, we have Valkia (good Power/cost/hp ratio anyway), Nurgle Sorceror (it's expenisve, but it's strong) and the funny Festering Nurglings.
We have useful effects almost anywhere.
BTW, I won lots of games with Orcs playing against them and it has a "hard" match up with Dwarves.
I don't say anything about Empire cause they're more similar than someone may think...I've to understand their match up yet.