I used Iknow in the past - both Yorkshire and Lakes - but dropped them after a while; this would be 4 or 5 years ago when OD were still a good site. They were very persistent.
After IK underwent big changes I had another look at them but didn't like their model, as they'd be too involved in how I choose to run my own business. They seem to be trying to compete head on with the big boys while only being a small player, which is unlikely to have much success - they don't have the $Ms to spend on Google. Be wary of any "guarantees" - the small print usually says something along the lines of "there are no circumstances under which we couldn't wriggle out of coughing up."
I no longer use OD, or indeed any of the major sites so can't be of much more help. But a key point is that one person's recommendation or condemnation of a site is irrelevant - they all work for some and not for others, and nobody here has yet found the magic formula for determining if they'll work for them! So far, the only true test is to sign up, cough up, and see what happens - bearing in mind that the following year could be completely different
There's a lot about OD on here, less (at least recently) about IK, so maybe some current users will chip in with their experiences, but IK and OD are not a straight comparison.