Sorry, couldn’t resist.
I started using PSF’s Vitamin C Serum recently, after much hesitation. (I had a bad experience with a MyChelle C serum breaking me out, so I’ve been more than a little paranoid!) While I know it can take up to six weeks to really see (“C”) any difference, I can tell that I’m already quite happy!
First of all, I haven’t had any breakouts or increases in blackheads. Many users on EDS have complained of an increase in blackheads after starting a C serum, and I think the generally accepted theory is that the vitamin C oxidizes the contents of pores, which is what makes them look black. 
Thankfully, my existing blackheads are no, um, blacker than before, and there don’t seem to be any armies of new ones. So that’s a real relief! And I haven’t had any changes in breakouts, either. Whew!
Second, this stuff doesn’t sting or itch, and it’s not too sticky. The PSF serum is in a hylauronic acid base, and I’ve been worried that HA stung my skin and caused some sensitivity in the past. But that doesn’t seem to be a problem — even when applying over broken skin, this stuff does no harm. And it doesn’t seem to react poorly with my Retin-A, either, which is another relief! The texture is ever so slightly sticky after I first put it on, but this goes away by the time I layer on another product.
Lastly, I’m thrilled to say that I’ve already seen some tangible benefits, and not just a lack of harm. This C serum seems (“Cms”? Too much?) to do a better job of exfoliating my already-flaky areas than Retin-A alone. Case in point: I have a pimple that was rather a pain, and had broken skin and developed into a scab. Using Retin-A around areas like that almost always results in some major drying and big, ugly, peely flakes. But once I started using the C serum, that area seemed less prone to major flaking, and it seemed calmed down and desensitized quite a bit, and seems to be healing faster.
All in all, I’m thrilled! I don’t love having to wait 30 minutes after applying this stuff before I can layer anything else, and I also don’t love how unstable topical C serums are known to be. But this stuff is packaged very well — both the sample and the full-size come in a dark amber glass bottle with a glass dropper, so very little light or moisture can enter the container to spoil the contents, and there’s no way toxins in plastic could leech in. So I think I chose a good C serum, at least for now — once I use up my (very generous, as it should be at $10!) sample, I will decide whether or not to repurchase. My gut is telling me I’m going to take the plunge, but who knows — we’ll have to wait and C! (C’mon, you saw it coming.)