It'd be hard to gauge this from someone's experience with a toggle AC since this is a new feature. I've played on plenty of very hostile PvP servers in RP 'settings' where you had to build smart & plan accordingly if you wanted to pull off a 'gank'.
Generally, for melee characters, you attack from stealth or invisibility to catch someone off flat-footed and gain an advantage to attacks if they don't have blind-fight, landing an Improved Knockdow to keep their AC down as you lay in the criticals or sneak attacks. In low level ganking low levels, standing IE AC would make a difference, but, low levels aware of ganking *would have had IE turned on whenever they were still anyway* so this change isn't too different. It makes less experienced players better at using their IE to block ganking, that's about it.
If you're a caster ganker that begins chain-casting empowered damage spells or death spells, being able to use IE as you run away (better that is, you could still do it before between cursor clicks) before triggering invisibility, greater sanctuary and/or hide in plain sight to attempt again at another date if the gank failed.
In mid to high level PvP, whoever is going to gank someone else will already prebuff their attack significantly before even approaching their target to make sure their AB exceeded their opponents AC, which is easy enough to do on low magic items servers and if your target isn't encased in armour 24/7. The unbuffed target with an extra +10 might offer a little more resistance, but not much, flat-foot penalties are harsher to some classes than others but they still do something even if you have blind-fight and uncanny dodge.
I think the only real issue that may arise, is people using IE while escaping in a much easier manner than before when toggling it back on between their running attempts.
In short, it's a great feat which always has its better use when you are 'not attacking' because then the penalty to attack is redundant.
Running away in Improved Expertise mode is easier, and it's not going to be interrupted prematurely now during combat. If you really want to gank someone, you can still do it easily, just don't skimp out on the preparations and complain after.