Incredible no-brainer.
No downsides that I can think of. In addition to lightening the trigger in both DA and SA, somehow the lower pressure also smooths the trigger (feel so to me). The trigger is so good, that at times I feel as though I could actually live with it as DAO.
And once again, no downsides: D spring does not increase the risk of misfires (going one notch lower to the even lighter Competition Spring supposedly increases the risk slightly) and the total risk exposure if you don't like the D spring is 5 minutes of your time and ~$10.
I tried the Competition spring also, under the same logic. Bought it along with the D, but felt that the marginally lighter trigger wasn't worth any risk of misfire for a self-defense pistol. 5 minutes later, the D spring was back in and I was confident that the set up was perfect for me.