Tom Pelissero makes some good points in his article about 1st round DT Justin Harrell. He hasn't played very well this season, and he's been spending more time lately with the second stringers and less with time with the starters. He's been outplayed this preseason by DTs Corey Williams and Johnny Jolly, but he's been about as good as DTs Ryan Pickett and Colin Cole.
The best thing about Harrell is that he's never played bad. He might not be creating much of a pass rush or leading the defensive line in tackles, but he's not getting shoved all over the field either. He is a rookie and although it might not take a long time to learn to play his position, he does have to get used to the speed and the caliber of players he is playing against. It should take a several games or even his entire rookie season to get used to it.
Unlike the list of the worst 1st round busts listed in the above linked article, he doesn't have any (known) off field problems to worry about. Plus Pelissero makes a good point that it should take some period of time for him to get back into football shape after not playing for approximately nine months. Although his injury happened at Tennessee around late September, early October, he didn't play in any of the minicamps (as far as I recall) and didn't get back on the field until last month.
He's not a bust and although he doesn't deserve to start, he's going to get playing time and improve as the season goes on, and will be an important part of the defensive tackle rotation by the end of the season.