WHMCS is in my opinion, the best system you can find to manage a web-design and hosting company. But the lack of QuickBooks integration made it a pain to manage end of year taxes. We tried a few different products and solutions, and a few months back we found MyWorksDesign's pugin. This is probably one of the best plugins that you will find in the market - period.
The top-notch level of customer service that I have personally received from Peter (The CEO of Myworks Design) is worthy of a 5 star review on its own.
The Product is well documented, has clear and detailed instructions, continues to expand with added features, and is backed by a team that actually uses the plugin to run their business - so they need it to work.
As far as what it can do: The product does exactly what is described. It syncs our clients, invoices, and any other type of pertinent data between QuickBooks and WHMCS.
This system is definitely worth its weight in gold if you just want a simple solution that you can set and forget, this is it.