I have read that while LED's will last a very long time the output diminishes over time until at the end they may only produce a 1/4 of their original output. Some of it it is the individual little lights deteriorate but some of it is some of them just fail. Don't take this as gospel. Do more research. May be dependent of the original quality.
I have also been told that there light and "apparent" light. In some cases to get what you perceive as a doubling in the brightness you must increase the output by tenfold. Clearly I need a better understanding.
I installed four 4' LED fixtures from Costco in place of two 8' two bulb T12 lights and am thrilled with the results. Great general illumination.
Read Bill's article.