Although you can't have a normal conversation with him, I find that he has almost as much to say as many of your other characters. Sometimes several missions will go by before I am able to do a new conversation with my other party members. Garrus is a prime example of this.

Zaeed, on the other hand, will have something new to say after every major mission, and most smaller ones. Usually it's along the lines of "oh, this last mission was such and such. It reminds me of the time..." and then he has a few lines talking about some mission he did way back when.

And I find that pretty believable - it fits his character very well, and, if you pay attention to him, he does soften up and start to become more personally invested as the game goes on.

And in every other way, he's just as fleshed out as the rest. He has a loyalty mission, he has plenty of lines scattered throughout the other missions if you happen to bring him with you, etc.

I thought he was a great addition. Never meant to be a star player in the story, of course, but he pleasantly surprises me.