Andy, I bought a CP910 earlier this year for my annual 'wander' through New Mexico. I bought it so that I'd be able to make prints on the spot for people I might meet and photograph (thought it would be better than the 'give me your email address and I'll send you a copy' routine). It proved to be a great ice-breaker, and all of a sudden I was everyone's best friend wherever I went and photographed.
Originally, I bought the Fuji Instax SP-1 mini printer (partially for its size, partially for its compatibility with the Fuji X100T), and after having had it for a month, I sent it back to B&H for a full refund. The image quality was so horrible, and the print so minuscule, that I felt embarrassed to hand one to somebody. Plus, at $172, I didn't think it was worth it. The CP910, however, is much less expensive, and the paper/ink cartridge is much cheaper, but add in the battery kit and you're up to almost $200 total. Plus, it is physically larger than the SP-1, but I'd gladly sacrifice a bit of extra bulk for a bigger, better photo.
Speaking of the photo, you wanted to know about black and white. Well that's pretty well all I used it for, and I have to say that the quality is quite good. It's not 'fine art' quality, but for something that you might give to someone, it looks pretty darned good to me and you won't have to make any apologies for it. The actual printing process is even entertaining to watch -- it's dye-sub, so it lays the image down in layers of colour. The print is pulled into through printer from the tray, and it prints the first layer (yellow, I think?), then it pulls the paper back in and prints the cyan, then back again for the magenta, and then finally the black. At first you wonder how the heck this multi-layer process will result in a black and white print, but sure enough it does. I think the total time for it to make a 4"x6" is about 40 seconds.
The printer itself has limited adjustments -- I think there's a brightness adjustment, and a couple of other things that I can't remember offhand. I found that it did print a bit darker sometimes, so I'd have to make use of that brightness adjustment to approximate what the photo looked like on my camera's LCD screen. As a result, the first print might be the 'test' print, and then the second one was the 'final'. So you might waste a bit of paper from time to time.
Two final advantages that this printer has over the SP-1: It has an SD card slot, and it has a USB outlet. So not only can you print directly from the card (make sure you are also saving your pictures as JPEGS in your camera), but you can hook it up to your laptop, make image adjustments in PhotoShop, and then print. Normally I was printing directly from the SD card from my Leica Monochrom, but occasionally from my X100T. Had I been using the SP-1 printer, I would not have been able to print from the SD cards, though it did offer good Wifi connection to the X100T.
All in all, I am very pleased with the CP910, and would recommend it for decent black and white 4"x6" prints.
Oh one final thing about ink/paper usage -- when you buy a pack of paper, it comes with an ink cartridge, and it's enough to print that pack of paper (nothing more). If you buy the large box of paper (the 108 pack), it will come with three cartridges -- each cartridge can print 36 prints.
