People's opinions are warranted by whatever reasons they see fit. You don't have to agree, but if they feel that way, they feel that way.
No one is denying Howard's talent. The man act.
Rhodey has always been presented as a counterpoint to Tony as a character. Tony is more of the brains, Rhodey the brawn. Tony is boardroom, Rhodey is street. Rhodey has always been a physical guy, and is a former US Marine. Terrence Howard has never come off as a imposing guy physically. In fact, many people on the boards have stated that Howard could be Stark if they were colorblind casting.
What I fear is that they want Howard because Rhodey will be depicted in the film as Stark's contemporary rather than a counterpoint. Of course, i'm just gathering this based on what we know right now. But if my assumptions prove correct, I have every right to be disappointed, as they have altered the classic dynamic that always worked well in the comics, and one I wanted to see on screen.
I'm not turning into a doomsayer just yet, but I am saying these casting moves have not instilled confidence in me about this project. Hopefully I'm wrong, and IM is both a faithful adaptation and a great film.