When it works right it disables the target for a few seconds and can partially or fully knock down shields.
Back when I had a Warlock back in 2.5 I didn't like it as it would also disable the ship firing the EMP i.e. you, but these days my Sentinel allows me to fire the EMP and continue flying... although you have to remember to turn it on using the ship weapons menu, it's off by default.
At this point in game development it is hard to say if the version in 2.5 was the way they wanted it and it's broken now, or if the way they want it working is as now and it was broken back then... Or if they even have a different way they want it working in the end...
I say Fly Before You Buy, try to get a play with one and see how you get on with it but beware it won't be balanced and finalised before Alpha, so don't get used to how it works now.