I dig them, they are pretty neat, and are not that humongous once you carry one for a bit. Probably a piece of cake for you since your used to lugging a 4x5 around. I have both the 90mm and the 65mm models from the series II. Both lenses are quite nice, I havent put that much use into the 65mm as I have only had it for less than a year, but I have shot a ton of rolls though the 90mm model. I think the 35mm focal length equivalences are 40mm and 28mm. Both I bought used with counters at around 200, one in from the US, and the other direct from Japan.
I chose the II model mostly for the looks, I like the boxier shape, and the texture of the grips vs the III's more streamlines plastic casing. The II incorporates the hot shoe which the I is missing. The III series has a convenient pop out film button which makes loading a bit faster. All three have the same lenses I believe. I use both cameras with the weight reducing optech pro strap, the stretchy neoprene really does help make it feel much lighter. There is also quite a price difference between the II and the III models, and much less of a price difference between the I and II models.
These cameras burn film like no tomorrow, I find that I reload quite a bit when using them as you have only 8 shots. 220 film helps with this though, and its not to hard to flip the pressure plate and flick the counter setter in the field but I wouldnt recommend doing this often as you are bound to mess it up one time haha.
They use 67 mm filters, I carry a set of 6 or 8 filters with me in a pouch. 67mm is not too bad in price now either and you can get good used ones easily since it seems that all the current lenses now seem to use monstrous filters like 77mm. If you use filters that are not screw (like conkin) in you might have to cut off the hoods from the II and III models to get to aperture and speed settings.
I had been printing the negatives cropped to 6x7 for a bit and then 6x9 with my homemade glass carrier, and now finally 6x9 with a slightly modified universal negative carrier on a LPL 670xl. I am guessing since you have a 4x5 setup you have an enlarger that does 4x5 so no worries with this. The negatives that I get are sharp and crisp, and enlarge very well, I have been in a 11x14 and postcard mode for a bit so I havent enlarged to 16x20 yet (which is the largest paper and easel size i have). The proportions are the same as a 35mm neg, and you have to crop or print wide boarders.
Do you utilize the the shift/swing/tilt of your 4x5 on assignments usually? If you need it there isnt really anyway to do without it, unless you start tinkering digitally and even then it might not be so great. But if they just want you to use utilize images from the Toy cameras, then this point is moot.
Might as well grab a camera and shoot for a bit, and then make your decision before you bring it with you on the trip. I think the prices that I have seen them going on the bay are the cheapest yet so far in a long time. I dont know where they are digging them up these last few months but there are a lot and have been selling quickly. Try it out thats the safest way, if you dont like it I think you can easily flip it.