First- they did do a tv miniseries on the Sci-Fi network back in the early to mid 2000's, which I thought was very good and still rewatch every now and then. It is a bit odd- at times it felt like they were filming a stage play with some of their choices, but it worked.
And for the record, CSTowle- I think that miniseries had the same problem you mentioned. For instance, Duncan was even less developed in that mini-series; he became a larger part of the sequel, which was based off another book.
Having just watched it at home (I'll go to the theater next weekend), I did like it. The visuals were stunning, and the performances were very good. I actually felt sorry that I both saw the Sci-Fi miniseries back in the day, because I knew the fate of all of the characters, so there wasn't as much suspense for me (I read the book as well, but it's been over 20 years and I don't remember it that well).
One thing I don't like- we never saw the Emperor or his daughter, and we never got the dinner scene to set up a possible romance between Paul and the Daughter. We also never got any false leads that Jessica would betray Leto, which would lead to Gurney being suspicious of her. That becomes a point of tension later in the story, and as I recall, the Emperor's daughter had her own subplot navigating the politics her father put into play by siding with the Harkonnen. We also never saw the younger nephew of the Baron.
My biggest concern is that WB won't greenlight a sequel. I feel if I could see the whole story play out in two (or three) parts, I would like it more. But if it ends here, and we don't get to continue the story, it's only a very pretty movie that I wouldn't watch again.