Well, I guess it is up to me to provide the contrarian viewpoint...
First, let me say that I am not knocking the guy. He is a playmaker and will clearly be successful.
HOWEVER... any talk of him being top 5 or top 3 is very premature, imo. The two most common comparisons made when talking about CJ is Westbrook and Tiki Barber. From what I have seen, both of those guys were more natural receivers and had better builds, that is, they carried more weight in their legs.
Westy and Tiki both put on some weight over their careers, and perhaps CJ can as well, but I am mildly skeptical. CJ has the look of a guy that isn't going to put on bulk. As such, he will never have the power necessary to be a goal line guy, which will limit his upside. It has been mentioned that he breaks tackles, but over 140 of his 251 regular season rushed went for 3 or fewer yards... and there were too many negative yardage carries. I didn't see a guy that broke tackles when I watched him. He broke some tackles, but he wen't down about as easy as most 200 lb players. Tackle-breaking is a function of size, density, and leg strength, and CJ has none of these, imo.
That leads me to my next point: CJ's rushing success will be somewhat dependent on his offensive line. Duh, that's true for all RBs. Well, Tennessee had a very good O-line last year, and that buoyed CJ's rushing stats. That should be true this year as well, but if CJ doesn't have those huge holes (and there were a lot of big ones) to run through, he won't have as many big plays, and that will lower his ypc and TD total.
Speaking of TD total, it's likely that CJ will be up and down from year to year. He had 10 last year, but only 2 of those were from inside 5 yards. It's hard to score from outside of 10 yards as a RB, that's just a simple fact in the NFL. Without double digit TDs, CJ will not be top 5. Frankly, he will likely need 14+ TDs to reach top 5; it is possible, but I would put the over/under for his TDs at about 7.
I also think CJ has good hands, but not Tiki or Westy hands. He'll get 50+ catches, but I think it's unlikely that he gets the 70+ that is necessary for him to leap into the top 5 RBs. One would think that CJ will improve on his 6 yards per catch, and he will certainly need to to get into the top 5. BUT... until Tennessee gets some passing game threats, teams are going to shadow CJ in the flat. This will especially be true if Tenn targets CJ a lot more; CJ is their biggest threat, and teams will treat him as such. Kerry Collins is no McNabb, and Tennessee's WRs don't even have an Amani Toomer level guy.
My last concern is from a dynasty perspective. He is a smallish guy, so there is always the injury specter lurking. He has decent elusiveness, but not the kind that will keep him from taking shots. More carries will lead to him possibly wearing down and that makes an injury more likely. I am of the firm conviction that ALL RBs get hurt, and the best ones play hurt.... but CJs game is built exclusively on his elite speed, so any nagging injuries can reduce his effectiveness more than a RB that is more well rounded.
All that being said, I fully believe Chris Johnson is a fantasy RB1. I would put him in the 7-12 range. I think about 90-110 yards per game is the right range, but lack of TD upside will hold him back. IF he can get into that 14+ TD range, watch out.
But I just don't see ELITE when I watch him.