I liked Luca. He was cool and really had love for the Family and Don Vito. Reading the book puts him more on a personal level and gets more into his personal life and makes you think he's like a big teddy bear (until a job had to be done, then he was brutal and had a bloddy mouth)
I wish he woulda lived.

Not having read the book, I can only comment on the Luca character from the movie.
I wish we could have seen just why Luca Brasi was so feared. Some wiseguys are power, others in it for money, women, the rush they get from controlling others etc. The vibe I get from Luca is that he liked his job, gruesome as it was and at the same time had intense affection for Don Vito.
I too would have liked to see just made Luca tick.

To be honest, the first time that I saw The Godfather, I pictured Luca Brasi as some weird oaf who was unsure of himself.

The first scene with him in the movie is him mumbling in a corner practicing a speech and then wrongly reciting it to the Don. But then we see him get involved in the family affairs and I have a sudden respect for him.. so I guess he's just one of those unique characters. It would indeed have been nice for him to survive.

What would his rank be in the family wiseguy,etc?

I don't think he could have survived. Don't forgot Solozzo's line about I could have never got to him in his earlier years.

Him giving up his best enforcer on what some would call a "stupid" move showed he was "slipping" I think for that reason he had to die.

I didn't like Luca Brasi at all. The Corleone family used him because he was strong, capable, and loyal, but they were afraid of him too.

Jackson, I am surprised you liked him after reading the book. It goes into detail many of the horrible things he had done.

I believe that he would have been a button man or the Hammer himself. He got all the dirty work done. I agree about him being vicious in the book- (SPOILER! SPOILER!) with the whole business of the Irish whore and all.