I have been using 3.5 to write code. I wrote a 'simple' app (200-300 lines) that although 'simple' it took A LOT of back and forth with the 3.5. In the end I had to spend a couple of days combing through it to add some key features (i.e. set values to zero so you don't see old data, etc.). Basically crude (but smart?) coding (I am not a professional developer).
When I finished the code and the app was performing EXACTLY as I wanted (v1.0), I thought to start planning for the v1.1 and when I pasted my code to the 3.5 and asked it to 'work on it on the new features' it was a shitstorm :)
Perhaps it was me feeding the requirements 'wrongly', but it was like pulling teeth 'guiding' it to correct even the simplest mistakes.