I found it slow. Also, just remembering this now but I came in to work one morning only to be questioned as to why I was downloading GBs of data while I was out of the office. I guess Dash was refreshing the local copy of all the docs every night. Uninstalled it right away.
This was my first time seeing DevDocs, it's awesome! I've used other all in one doc solutions but I always end up going back to google because they slow down once I add all the languages and packages I use.