So basically you are asking what we think of a certain visual novel brand.
For Key, I have read 3 Key visual novels, Clannad, LB and RW. What I noticed about key is that they add the visual novel gameplay, i.e. repeating the routes into their plots or to say, you are repeating the game as a part of the story. Usually they also add a comic relief buddy to the protag. Though Key produces great games, they are somehow overrated IMO.
For AB2, I have read 2 and will be reading one after finishing what I am reading now. AB2 usually brings out something like that dark part of the society in both VNs I read. I say that AB2 usually deals with seriously stuffs like society and politics. This is one of the reason why I love and hate AB2 at the same time.
For Feng, thay usually have a typical slice of life harem game but they do excel when it comes to art. It's sad the no translation group has picked up a Feng visual novel for translation.
Eushully produces more gameplay heavy games IIRC. I was only able to play one. It takes a lot of time but is definitely worth the time and effort to complete everything.