Just a word of advice - if you are going to buy either the Apex trigger or the spring kit - get them both. I bought the spring kit thinking "oh, if this isn't light enough, I'll just get the trigger afterwards and install it later!" Bad idea. The kit was a pain to install, and the springs alone helped but not as much as I had hoped for. I ordered the trigger, but now it's just sitting on bench, waiting for me to install it one day.
I'll get around to it, but damn. I am not a handy person mechanically, and it was like a 6 hour adventure last time getting the SD9 apart and back together, including at least one of those hours where I lost a firing pin cup in the carpet and had to go looking for it (and happened to find out that the SD9's are the exact same size as a Glock's). I could just pay a gunsmith to install the trigger, but it would more than double the cost of it, and I'm a cheap SOB. But then I'm also a lazy one, and a clumsy one...
You get the picture. It's a job you only want to do once.
Along that adventure, I found that a lot of the internals of the SD9 are plastic where the same ones in a Glock or M&P would be metal, and no doubt that will lower the lifespan of the gun when it comes to extreme, five-digit round counts. But they should last for a very long time on any normal shooting schedule.
I really have no complaints about them besides the trigger, and the fit and finish is shockingly good for a $300 budget gun.