Portis has a pretty tight spread of what his stats will be, but that's a double-edged sword. What I mean is that barring a significant injury, he will likely end up with numbers that put him squarely in the top 10 of fantasy RBs, but it's doubtful he will crack the top 5 range. So he's good in that he can reliably put up good numbers, but he's bad in that he's probably not going to put up "blow your mind" type numbers. He's not going to be the reason anyone wins their league, but he's probably not going to be the reason anyone loses their league, either.
People get excited about guys like ADP because they have the ability to put up numbers that are so big that they can put your fantasy team on his back and carry it to a championship. But here's the other thing about those guys, if you take them in the first round, you get screwed if they don't put your team on their back. It's the guys that get taken in rounds 2-6 that win championships because they outplay their draft positions, not the first round guys, because the first round guys can't outplay their draft positions, the best they can do is meet expectations. Last year, Randy Moss, Tom Brady, and ADP were the big names because they were being taken in the third round or later for the most part. They all put up first round numbers, so teams that had them effectively ended up with multiple first round picks, but here's the thing, if your first round pick is a bust, having someone step up from the 3-hole just gets you back to even, not ahead.
To bring it back to Portis, his reliability allows you to take shots on the risky guys throughout the rest of the draft because you don't worry about what kind of numbers he's going to put up. He's the guy that can produce up to his first round draft position so that any gems you find put you over the top rather than just even.