I've used Clockk for the past year (I had beta access) and it's made time tracking easier and more accurate. I tested my old system alongside Clockk, and found I was underbilling by ~$60 a day on average before switching to Clockk.
I'm a designer with multiple clients, so I find myself task switching many times a day and normal tracking methods would miss the smaller tasks unless I took note of them. Stopping to set a timer or write down a small task kills my flow, so having Clockk just automatically track what I was working on, when, and for how long, so at the end of the day I have an accurate, visual representation of what I did has been amazing. It takes me about 2 minutes at the end of every day to quickly review and then submit, ensuring I'm tracking and billing all those little 15 minute tasks which normally would fall through the cracks and not be billed. Once I saw that it actually made me money, I have been a daily user ever since.