I've gotten into instant film pretty seriously in the past year, via a Polaroid Land Camera Model 100, Colorpack II and Fuji PA-145 film back on my 4x5 camera (for the Fuji pack films) plus a Polaroid Model 600, SX-70 and two Spectras (for the Impossible Project films). The cost of the Polaroid cameras ranged from $6 for a Spectra to $75 for the SX-70.
The Fuji pack film makes nice images and isn't too expensive. The old 1960's and '70's cameras usually work pretty well, although it takes a while to get used to their primitive autoexposure. I've tried drying, cleaning and scanning the negatives from both B&W and color and they're fun to experiment with. Taking a photo with pack film is a deliberate process though, especially if you're worrying about saving the negatives.
When choosing an automatic Polaroid pack camera, I would recommend the older rangefinder models with the "Scene Selector" switch on the front. This allows full aperture of approximately f10 in low light situations, which works very well with the Fuji FP-3000B B&W film (ISO 3000).
The integrated Impossible films avoid the muss and fuss of goopy negatives and waste paper, but this convenience is offset by the 40-60 minutes it takes for an image to fully develop. This is not Instant Photography, it's a One Hour Photo. The slowness makes it very hard to make compensating adjustments in exposure or lighting, plus the $3 cost per print is a serious disincentive to experimentation. Generally, the colors in Impossible prints are somewhat murky, with a yellow or blue-green cast overall.
Films for the SX-70 and 600 cameras (PX type) are prone to streaking and blank spots because of uneven developer spreading. The SX-70 that I have is an early model with manual focus and indeed it's a wonderful mechanism to unfold and behold. However, the camera was originally designed for ISO 100 film and the PX-70 film is actually quite a bit faster than that, so you must remember to adjust the exposure control wheel in the 'darken' direction whenever you open the camera. My Model 600 camera is awkward and clunky and the flash always fires no matter what, but at least the PX-680 film is the right speed for that camera.
I've been much more pleased with the Impossible film for the Spectra cameras (PZ type), both in the wider aspect ratio of the photos and in the more consistently acceptable color and exposure. Spectra cameras date from the 1980's and '90's and have sonar autofocus, sophisticated autoexposure and variable flash. They come in several versions, the most useful type being the original "Spectra System" models with the switches on the back that control functions like autofocus and flash override. The later "Spectra II" cameras lack these switches, but otherwise perform well.
I haven't tried the Impossible B&W films yet, but they are reportedly very fussy about temperature and need to be carefully dried to ensure long term image stability. Overall, the Impossible Project films are a work in progress, but their image quality is steadily improving.
No matter which Impossible film you use, be sure to purchase the appropriate "Frog Tongue" accessory that fits your camera. The emerging print is very vulnerable to light when it first zips out of the camera and keeping it covered with something like the Frog Tongue for the first 5-10 seconds is very important for final picture quality and contrast.