Sonder:
n. the realization that each random passerby is living a life as vivid and complex as your own—populated with their own ambitions, friends, routines, worries and inherited craziness—an epic story that continues invisibly around you like an anthill sprawling deep underground, with elaborate passageways to thousands of other lives that you’ll never know existed, in which you might appear only once, as an extra sipping coffee in the background, as a blur of traffic passing on the highway, as a lighted window at dusk.
source: http://www.dictionaryofobscuresorrows.com/post/23536922667/sonder

Considering Alex Mercer and the Evolved consume people and gain their memories, do you think that this is a common experience for them, or do they have some kind of mental block to prevent it?

5081612
Mercer probably has, but I bet he does not care, considering how much he killed.

Mercer became a Blacklight itself. He/it probably doesn't have any humanity left in him/it when the real Mercer was killed at Penn Station. As for Heller, his desire for revenge and to see his daughter is what he probably uses to block out all those other emotions.

I hope I've helped.

5081612 Probably a lot more vividly than anyone else, since they feel their complete unique lives each time they eat someone. The only question is if they care about it. "Yeah, they got a complete story of their own, but so what? Everyone does."