When I started figuring out what I wanted to do in PVP I decided to try ECM. The first ship I lost in a war-dec was a Kitsune. I've since flown BlackBirds (BB) and Falcons on several occasions, sometimes even in a fight. I thought I'd take some time to talk about the strengths and weaknesses I've seen in the ECM ships in case my experiences can be of use to someone else. Hopefully there will also be some comments to point out ways I can improve my understanding of ECM.
First let me start with the Kitsune. The Kitsune is the frigate sized T2 ECM bird and has 20% bonus per level to ecm strength and 10% bonus to capacitor use. With level 4 skills the Kit has 80ish km targetting range, optimal of 76km for racial jammers, 9.5 strength for racial primary, and 3 strength for racial secondary. The EAS is a great choice when you need fast lock and are generally with a fast moving fleet. The problem with the Kit is the same most frigates are susceptible to, they just don't have the staying power for big fights. The Kit is the little brother to both the Falcon and the BB. Having the lower jam strength of the BB and the shorter range of the Falcon ends up making the Kitsune a poor choice for combating bigger ships. Recently I took a Kitsune out on a cruiser roam and it did fine as long as I had some range to work with, but it died in one volley when I accidentally landed on the enemy fleet.
Next up is the Blackbird. The BB is great when there is room to maneuver or you want to use a cheap throwaway ship. The BB has a 15% bonus to jamming strength and a 10% bonus to optimal and falloff per level. The fit I use has the BB's optimal at 89km, fall off somewhere past 150km, targetting range out to 121km, jam strength of 9 on the racial primary and 3 on the racial secondary. I've used fits that had closer optimal and shorter targeting range but they ended up dieing rather quickly. The bonuses to the BB indicate it's meant to work out at range and it pays for this distance by sacrificing jamming power. The BB is great for supporting BC and below sized fleets as long as there is room to maneuver. The BB requires at a minimum 50km of range to keep it alive and not all fleets or roams can provide the necessary distance.
The last ECM ship I have experience with is the Falcon. The Falcon is unique in that it can fit a covops cloak and can fly around ninja stealthy. It also gets a 10% bonus to jammer capacitor use and 30% to jammer strength. These bonuses give the Falcon the highest jam strength of the ECM birds but pays for it with the shortest range. My fit has 150km targetting range, 68km optimal, strength of 11 on primary, and 3.7 on secondary. The ability to cloak and manuever to the ideal position before revealing oneself often makes up for the Falcons poor range, but once the Falcon is spotted it has to flee or die. I see the Falcon as more of a small gang ship where the jamming strength provides safety and doesn't have to rely on staying out of the enemy's ability to hit.
There are three other ECM ships that I have no dealings with; the Rook, Scorpion, and ECM fit Tengu.
The Rook is the combat version of the Falcon with the same bonus but lacks the ability to cloak. Instead the Rook gets some good missile bonus and is tougher overall. I think the Rook is designed for the bigger fleet fights as it has the staying power at shorter range.
The Scorpion is the battleship sized ECM ship and has bonus to jamming strength (15%), jamming range (20%), and is the only ECM ship with a bonus to ECM burst modules (20%). This gives the Scorp roughly the same ECM power as the BB but with even more range. The Scorp seems to be best with sniper fleets or equipped with a massive tank and sent into short range to fire off burst modules.
The final ship is the ECM Tengu which has a 10% bonus to jamming strength per level and 10% to optimal/falloff. These bonuses provide the Tengu some decent ECM power. There should also be several medium slots available for both ECM and tank so an ECM fit Tengu should be a capable foe in small gang warfare.
Overall I prefer the BB as my ECM ship of choice. Cheaper than both the Kitsune and Falcon, versatile enough to fit a small tank, capable enough to engage four targets, and lower training cost than the T2 ships for maximum bonuses. The BB is a great starter ship for anyone interested in ECM offering an ideal combination of power, affordability, and training opportunities.