I've used it from time to time, but like most aids to route planning, it needs to be used with care, and more as guidance than as an entirely suitable route.
I tend to use a mixture of it, Google Map's route planning (now with bicycle routes, which are often ludicrously longer, and involve illegal use of footpaths etc), and the TfL route planner cycle option.
All of them can give some strange results, but CycleStreets is generally better than the rest. It did once give me a route which would have involved crossing a railway line using a footbridge to save a few hundred feet (but used a significant diversion, so a change when I realised would have been costly distance-wise). That would have been mildly irritating usually, but I was towing a trailer, and it would have been disastrous. For that sort of check, Google Street view can be a great help, since it lets you clearly see what is involved for the less traditional routings (ie not on a road shown clearly on a map).