Thought I'd look at our selection of Mendenhall over Johnson in the draft a few years ago. Pretty much numbers and my own feelings, and spurred by this quote in one of Blitz's pieces today:

Ben needs to get the ball into Mendenhall’s hands all the time, any which way, by air or by hand off, he’s a ferocious athlete and I love his selection more and more every year.  -- TV'sChachi

Below are the main stats for both Johnson and Mendenhall.

Chris Johnson: (career stats)  Age: 25  Height: 5'11"  Weight: 191

Games: 39  

RUSHING STATS:

Rush Attempts: 787  Rush Yards: 3,955 YPC: 5.0 TD's: 31

RECEIVING STATS:

Receptions:  115 Yards: 853 YPR: 7.4  TD's: 3

 

Rashard Mendenhall: (career stats)   Age: 23  Height: 5'10"  Weight: 225

Games: 28  

RUSHING STATS:

Rush Attempts: 429  Rush Yards: 1,868 YPC: 4.4 TD's: 14

RECEIVING STATS:

Receptions:  41 Yards: 371 YPR: 9.0  TD's: 1

I'm going to play Joanie to TV'sChachi on this one. 
I LOVE Mendenhalls' selection, and often wondered why some people were dragging him by the collar to the dog house so soon. But, then I realized...I've done it before with other players, and so has everyone else on this blog, it seems like it's an old past time. 
So with that aside, I'm stoked we have him, and I'm very glad we have him and not Chris Johnson. 
My reasoning: 

• Toughness scale:  Mendenhall  > Johnson
• Steeler Style/Mentality: Mendenhall  > Johnson
• Age: Mendenhall  > Johnson
• Playing style: Mendenhall  > Johnson
• Size: Mendenhall  > Johnson
• Attitude: Mendenhall  > Johnson

It's rare mendenhall has a truly ineffective game, and with his age that sample size of games will increase. I don't see, with experience and study, how he will decline in any way in the foreseeable future.
His toughness is not an issue, and other than getting his shoulder busted by Ray Lewis, he's had no health problems. When he runs, he does so with authority, with vision, and with an eye downfield. He rushes hard, aggressively, and isn't afraid to lay a hit on someone.
His attitude is phenomenal, he's quiet, seems like a good person, and has no issues dogging him off or on-field. Other than his statements prior to the Baltimore game where Lewis murdered his shoulder (oops, did I say that?), he has not popped off or done anything to draw attention to himself negatively.
In a nutshell, I wouldn't trade Mendenhall for Johnson unless they included their first 4 draft picks. I love Mendenhall. I think he just fits us, and not sure I'd want the glitz and glam of a smaller back in there in Mendy's place.
Just my random ramblings on our stud RB.