実況s are awesome, they were my main source of Japanese exposure for quite some time. In response to why you would enjoy them, I say it's because of the context and the community feel. On Nico Nico you have the comments that appear at the point in the video they're posted, which makes it feel like you're watching it with others because you see people's comments for specific things while you're watching.
Something funny happens and people laugh and repeat the joke, or something surprising happens and people discuss it.
This also provides so much context for learning -- you have the video (game footage), audio (player commenting), and text (viewer comments).
I'll list part 1 link compilations of some favorites.
つわはす
http://www.nicovideo.jp/mylist/11694132
Calm and polite. Seems to be lucky all the time. I loved his playthroughs of the Crash Bandicoot and Banjo & Kazooie games (hadn't even played the Banjo/Kazooie ones myself before). Those were a real sense of adventure. I've watched each of the series more than once respectively, effectively making them better than TV shows.
セピア
http://www.nicovideo.jp/mylist/13917628
Polite but sarcastic. He's played the Zelda games and perfected everything in them, if you like those. He played an awesome and hilarious party game called 皆のリズム天国, and a series of cult games called 勇者のくせに生意気だ that I loved.
Kiyo and Fuji
http://www.nicovideo.jp/mylist/15923690
These guys are just absolutely hilarious. They're like a comedy duo, everything they make is humorous. They notably play a variety of mario games, including fan made ones, that they get a ton of views for because they're so funny.
Edited: 2014-05-31, 7:32 pm