Hey LD. I believe that the most stable films they have are the newest versions of the PX and PZ Silver Shades. Their new color film looks promising and I am getting a pack of that tomorrow, most likely, so we'll see how that goes. I havent used any of the Silver Shade with the SX70 yet but its the film thats been around the longest so Im guessing its the most the stable.
For the 600 and Spectra series cameras, the PX600 and PZ600 films are very stable. You can get some strange effects in the film, like some circles and some weird lines that look like webbing of some sort, but I personally love those effects. But over all the tone of the images, when exposed properly, come out pretty consistent. They start getting orange if you shoot in less than desirable conditions. At least with the first flush films. And dont be tempted into buying that film because of the low prices. Its not worth it.
The new UV film is pretty iffy still, though you can get some gorgeous results and you can edit them if you want, which is what I did. I'll post a few examples of the PX600 and the PX600 UV. But I found in low light the film gets these really ugly...I dont know, drip marks or something. Looks like the film has dried salt on it or something. Like you dropped it into the ocean and the water dried and left salt behind. I find it ugly myself and not easy to clean up so I wont be getting that film again until they fix it.
And yes, the film is extremely light sensitive. What I do is keep a piece of black poster board in my left hand when taking the pic. You have some time before the image actually ejects to get the poster board over the area where the film comes out uand then I just grab them together and wait for a minute and then stick the image in the foil lined box the film comes in and go off to shoot my next image. Its really not a big deal. You just have to make sure no light hits the image for the first 10 seconds, but its best to keep it hidden for a minute.
The first image was taken with a One Step using their newest version of the PX600 film. I did very little editing on this image and what you see in this image as far as tone and contrast and special effects are pretty much what I got with every image in that pack.
The second image is with the One Step and the new UV film. Too light for my taste but still likable.
The third image is an untouched version of the UV film taken with the One Step and you can see those salty lines I was talking about.
The fourth image is the same one, just retouched.
Hope this helps!