We've used Desk.com for the past 2 or so years. For the most part, it has met our needs. We built 3 separate knowledge bases for 3 separate products, used macros to speed up answering common questions, and integrated third party phone and chat apps so we can keep records all in one place. That being said, we're probably going to be switching away. A few reasons here. First, it seems like we encounter bugs, crashes, and more serious service-level disruptions too often. Second, it is pretty expensive compared to other products out there.
We came to Desk.com when we were really small. Our CTO was still the only dev we had so we were looking for an out of the box solution that could be customized by someone who was relatively non-technical. Desk can pretty much handle a lot of this customization but what results is something a bit clunky to use. This is not a criticism of Desk itself, more just an observation about tools that promise customization without dev resources.
Anyway, I think we'll switch to HelpScout. I like that HelpScout defaults to sending plain-text emails, where Desk emails look like they come from someone using support desk software. Plus, they have all of the essential features we're looking for at a better price.
Plus, I love HelpScout's content. I'm sure their marketers will love hearing this, but all their blog posts and resources give me the impression that they are a thoughtful company that understands customer support. It's always surprising to see how many customer support tools offer horrible customer support themselves. From my experience, that disconnect does not bode well for their product.