I think Preacher is a very polarizing book. There are people whose opinion I respect who laud the book, and people whose opinion I respect who pan the book. It's one you have to check out for yourself and make up your own mind. I did not like it and could not get past volume 3 on any of the three attempts I made to read through it. I haven't been a fan of much that Ennis has written except his war stories and (sort of) his run on the Shadow for Dynamite. Red Team had some potential but fizzled out for me. I won't saw Ennis is a terrible writer, but he is a flawed one, but so are many who produce work I like, so I will simply leave it at most of what Ennis writes is not to my tastes.
That's how I feel as well, I don't look down on anyone who like's Enis' work I just feel as if he isn't for me. It's not so much the levels of gore and violence he uses in work that turns me off, though those certainly aren't some of my favorite literary tools, it's usually just that I don't find the subject matter all that interesting. It's the same with most Western genre comics, though I love a good cowboy movie I can rarely get behind a western comic even if its done by someone I like.
Also, I love reading about why people love certain books, even if I might not feel the same, so I'd love to see why you and Slam love Preacher, Mars so I wouldn't let yourselves feel like it wasn't worth it.