I'm the guy that's started in on the ECM route as well as cruisers/BCs. So far, they've been
extremely useful at various times. Here's a quick list of the module types and what I've seen them to do. More experienced guys, chime in if I miss anything.
ECM burst: short range burst of energy centered on your ship, breaks locks on you. Can't be used in empire space near gates or stations, as it can hit friendlies.
ECM: long-range module that works on a single targeted ship. Breaks locks on you.
Sensor damper: long range, single-target module that vastly slows down the other guy's ability to lock on to you or anyone else. Best used with some form of ECM to break his lock and then fuck his ability to lock back on.
Tracking disrupter: slows down enemy turrets and cuts their effective range. Long-range single-target module.
Best used out of range of return fire.
Some of these modules come in racial varieties, meaning they work best against certain race's ships. I've not got the chart of sensor types in front of me, but a quick look through the marketplace and infoing each race's ships will tell you what you need to know. Just look for the sensor strength section in the ship info. Example: if a race's ship has mostly gravimetric sensors, use a gravimetric jammer against it for best results.
Sensor strength isn't just a straight up comparison of numbers. So far as I can tell from the forums, the various sensor strength numbers now reflect percentages. Example:
You have a jammer that's got a jamming strength of 4 for all sensor types. You try to jam a ship with a 10 sensor strength. Some math in the background later, it works out that you have a 40% chance of breaking through his sensor strength each time your jammer module cycles.
Note: ECM modules eat a pretty fair amount of cap. And since most ECM mods are mid-slot items, space for putting in Cap Rechargers is limited. You can put power managers in your low-slots if you want to run a few EW mods at once. You should also train up the various electronics skills to cut back on the cap use. You will also need a good deal of CPU to run more than a couple of EW modules.
Tracking disrupters are, I've found, of little use against missile ships. For those you'll be a bit better served by some form of actual ECM to break his lock on you. Sensor dampers can help too. Against turreted ships, though, tracking disruptors
REALLY cut down on damage output. I found it worthwhile to outfit one on my mission-cruiser for the big 'boss' rats at the end of each mission.