I've been doing C-41 with the press kits for about a year now. Love it.
Here are my observations:
Be careful what temps you develop at. Cooler (under 102f) is way better than over. Also, check your temps against more than 1 thermometer. I used a metal dial one and kept getting bad results, then started using a mercury one. The dial was a full 2 degrees off so I was developing at 104f. Hence why I know hotter is worser!
Dark brown negatives indicates too hot developing, or too vigorous agitation. If you see any streaking around sprocket holes, you may be agitating too violently. I was.
Now I just pick the tank up and swirl it around for 5 secs. My negatives have been darn near perfect ever since, coupled with the temperature revelation.
The developer seems to be the weakest link. It goes bad long before the blix. I now use a plastic bottle and squeeze almost all the air out before recapping. That seems to help a lot. The developer will darken as it ages. If it looks blackish, it's no good.
The stabilizer must be completely dissolved in water, use warm water to mix it. I also add Photo Flo and it helps a lot. I keep Edwal's negative cleaner nearby for the streaks or spots that come up.