It's quite fast, the instructions and explanations are very clear, it does what it says beautifully, it has limitations in what it can do, the folks there are nice. They have a free download that's only slightly crippled and allows you to try out all the features.
My biggest complaint is the lack of HTML reports for clients. If you want your clients to view reports, you must deliver those reports to your client by creating PDFs and sending manually. It's not realistic to ask your client to install their software.
An example of the limitations I referred to is filtering. Clicktracks allows you to filter out certain IP addresses and filenames, but you can't filter on browser names, authenticated users, activity on certain days, specific status codes (like 404's), visits from certain referrers, visits that started on a particular landing page, and so on. Nor does it have a way to filter in certain things, such as "only report on visits from x referrer".
If I'm wrong about the above, it's with good intentions, and please correct me.