Just purchased Rocksmith cable today, plug that into my Lenovo L440 laptop, it installed itself automatically with NO hassle and soon after that Sonar (in WASAPI exclusive mode)  recognized it as a possible input. So far so good. Inserted audio track, set its input to "Rocksmith USB adapter", switched on "input echo" and audio is coming, hooray. Adjusted latency to 12ms, inserted TH3 and audio comes as expected, good sound and decent latency, good job !
 
Then I armed track, hit record and :-(    ,now audio turned into horrible metallic resonance, recorded clip gets shortened by nearly half (and speeded up that much too). There must be something wrong with the Preferences settings I presume. Any idea ?

I had exactly what you experienced at first, but now it seems to work properly no matter what I do (Exclusive, Shared, share driver with other programs, etc.). 
 
Try a restart first with the cable plugged in. If that doesn't help, restart without the cable plugged in, then plug it into a different USB port.
 
I did make sure the sample rate setting in the Sound portion of the control panel matched SONAR's, don't know if that made a difference or not.
 
Don't worry, it WILL work. If you find a repeatable way to make it not work and then work, let us know your findings.

For me, it is all about the USB port. I got the same results as the OP and tried rebooting and setting sample rates, buffer sizes, driver mode, and Playback/Recording device settings to all possible values. Nothing worked. But moving to a different USB port made the difference.
 
I have two USB 2.0 ports on the top of my computer case plugged in to a Gigabyte motherboard. The left USB port results in bad audio but the right one works fine. The Rocksmith USB adapter only supports 16-bit, but when in the correct USB port it always works!
 
I am using 10ms buffer sizes with WASAPI Exclusive mode and my Presonus 22VSL for output (but I think it also works well with the motherboard's Realtek chips).
 
James

Indeed changing USB port helped! Playback and record goes normally. Sonar does not start with Rocksmith in and looks almost as if it crashed at launch,  but would start as soon as I remove Rocksmith. WASAPI goes to the top of my favorite updates list. So far I have not noticed any sound degradation due to "perhaps not so professional level AD conversion" that happens somewhere inside Rocksmith. Thanks Craig and JimG !

The quality of converters these days is such that even the low-cost ones are good...and the expensive ones are stellar. Also guitars have more level than mics, so preamps don't have to work as hard. So I think you'll find the audio quality of the Rocksmith cable will definitely do the job, especially if you're going to an amp sim that will have far greater influence on the sound than a cable ever could.