Don't know that I've gained a lot of hard/useful knowledge necessarily, it's more an occasional source of interesting news and informed commentary, within the subset of interests HN revolves around. Have gathered some background material on specific topics of interest to me (HCI, workspaces, personal knowledge management).
I don't submit much or engage in discussion very often (this post being an exception, trying to move the consumption/creation needle a bit), but HN is an excellent resource. The simplicity of its UI and general model is refreshing, and the signal-to-noise ratio and quality of both contributors and moderation is very high.
There's also a zeitgeist aspect; I know if anything of significance happens within a range of interests, I'll likely hear about it here.
Overall though, HN is more a source of news and intellectual stimulation for me than a learning resource. It's an excellent and pretty unique one though, would miss it if it was gone.