Just took Step 2 yesterday, so I won't be able to give an accurate answer until I get my scores back, but I'd say QBank was a good prep tool. Definitely not enough on its own because you'll get sucked into Kaplan's way of asking questions. I actually had a few questions on the real thing that I only knew because of Kaplan. Random stuff that I missed on QBank and thought there is no way in hell this kind of stuff will really be tested. Do some other questions like the retired USMLE questions or Appleton & Lange or something to get exposed to different ways of asking the same stuff. All in all though, I'd say QBank was probably pretty much on par with the actual exam as far as difficulty. That's just me though.
Good luck.