I hear a lot about Q/kdb+. I've never had the opportunity to use it for anything real but have played with it using their trial license and found it intriguing (if not somewhat mind warping). I've seen a few references to the J language which is also a derivative of APL according to Wikipedia and have heard of at least one quant trader using it as a poor man's replacement for Q/kdb+.
Has anyone put J to use in the real world? Is it worth investing time and energy into to learn, especially if I'm not interested in investing the dollars required to get a kdb+ license? I suspect a lot of K/Q's value comes from the kdb+ database that underlies it. I'm not sure there is an equivalent for J, but I haven't performed an exhaustive search.
Interestingly enough, J recently went fully open source and is now hosted (not officially) on Github: https://github.com/openj/core.
EDIT: Assuming some folks here are familiar with J and Q/kdb+ but have not used J extensively in industry, what might be some major advantages or disadvantages of using J.
EDIT 2: J has a database solution in JDB. One obvious follow-up which I also asked in a comment to @chrisaycock's answer below is how JDB might compare to kdb+ in terms of functionality.