Iantau: So can you carry two replacement film backs pre-loaded without problem... just remove an insert? or do these not work this way? Do you like your SLX's (Model 2 perhaps?) and are they new-to-you acquisitions, or purchased long ago "as new"? Preference to the later model 6000 series? What lens (or lenses) do you use? Do you have any size/weight experience comparing the SLX set up to a Bronica or Hasselblad?
Yes, the back stays on. You flip it open, take out the insert and replace it with a pre loaded insert. The exposed film can stay on the insert and go into the pouch. The inserts are designed to keep the finished roll from 'unrolling'. But I haven't really done it yet, I only know that the rolls look stable as I take them out. I guess next time I shoot Fomapan (PET base) I'll have to try and see what happens when the film is in there for hours with all the vibration and shocks.
I have five inserts. One in each camera without a pouch for it, and three spares in a pouch each. Of course that takes a lot more space than taking a JCH 120 5x film case with you and load on scene.
I took some pictures, maybe that will help:
Insert with pouch. I put in a dummy film (Disclaimer: that is backing paper only, no film was harmed or tortured for this). The start line is visible on the other side right in the middle by the markings on the insert. Second picture is the open back without insert, third with insert. The insert is symmetrical, so that you can keep the empty spool in place. Just place a new film in the empty position and replace the insert such that the the fresh film will be in the top position. Close the back and hit the shutter, usually twice, and the camera will forward to the first frame.
I only started photography as a hobby in 2013 using the digital Olympus system. Before that I was simply a snapshot guy without any knowledge about the technicalities of photography (aperture, film/sensor speed, DoF, etc.). I started with 35mm film in Autumn 2015 and found the first SLX (rev. 1) on Ebay during the Christmas break. I had no prior MF experience, and so far I never physically even saw any MF camera except my own. I bought two 1920s folders a year ago (Ica 6x9 and Rodenstock 6x12) but haven't used them yet. They need CLA of lens, shutter and that 'brilliant finder' (English term, anyone?).
The 1st rev SLX is roughly as old as I am, and I believe that the Ebay seller was a professional photographer, who offloaded his system. It came with an 80/2.8 Planar, a good and a bad battery and two inserts for €290. I also bought a 50/4 Distagon, a 150/4 Sonnar and a a 250/5.6 Sonnar. The lenses were all good deals. The older lenses are often cheap, but people do try to find a fool sometimes.

The later PQ lenses (can do aperture priority and maybe some other tricks with the later 600x bodies) and especially the PQS (1/1000s shutter) can be very expensive. The Schneider lenses cost ridiculous money.
The 2nd revision one was from an estate sale on Ebay. Some well off person must have bought a complete system in the beginning of the 80s. I payed €400 for the complete set in auction. Carry case, camera, 80mm planar, 2x teleconverter, Prism viewer and vertical viewer, a Gossen Lunasix F, batteries, two extension tubes, batteries, film inserts, an extra 6x4.5 back (not used it yet) and some other stuff. My most lucky buy on the bay so far.
Both cameras work fine, but I had the occasional hickup. Getting to know them I think I can handle/avoid those problems. I've had a few frames that were overexposed to near black. It might be the button on the cable remote. The camera needs a clean signal. It seems to be best to push the button firmly because a few times I had the shutter release not operating all the way through. Maybe the electric contact was to short or too weak. I'm not sure what is actually happening. Shutter or aperture blades click and stop there. Maybe the leaf shutter will stay closed or will stay open until I properly hit the button again. I'm not sure. When it happened I forgot to take a look into the lens to confirm which it is. It is a rare occurrence. I'm not even sure it resulted in a bad exposure each time. I'm too lazy to log each exposure all the time.
The older remote has corroded contacts on the connector. So I need to wiggle the connector and bring it into a position where it has proper contact. It's a DIN connector, most likely I could replace it with a new one. The newer, and smaller remote, from the 2nd revision camera, has nice blank contacts. You need the remote for mirror prerelease. A mechanical cable release can also be used for the shutter itself. You also need it for bulb exposure. Unless you want to keep the button pressed on the cable remote. Some later 600x cameras have a pre release button on the body, I believe.
I have also used the camera with near dead battery. Initially when I got the first SLX because I didn't have working charger yet. In cold winter weather the near empty cells couldn't handle it any longer. Eventually, as I forced my way through the film, the shutter opened and that was it.

The other time was when the battery was finally flat after 9 months and I was out shooting. I found my spare one self discharged over the same 9 months. Something you need to know about Nickel batteries. All avoidable, once you know. Some of the frames lost to overexposure may have happened there. I'm not sure because we're talking about 4-5 frames since I started in Jan 2016.
I prefer external incident metering. But there is an internal meter, which tends to underexpose when you have bright highlights or sky in the frame. In simple, low contrast light I had it happen that the internal meter and external incident meter show the exact same exposure. The internal meter is in the body and does not need a prism. The internal meter also helped me recently to measure the exposure difference using a polariser at a given angle. I metered internally with and without filter and applied the observed difference to my external incident reading. Spot on!
Whenever possible I use tripod, but walking around, shooting handheld with the 80mm is perfectly fine. I keep the shutter at 1/250 or 1/500, but was forced to use slower speeds sometimes. Portra 800 is great for handheld while having plenty of reserves, 400 films are still perfect. And I accumulated lots of Provia 400X last year. I still have 68 rolls, now, for handheld roaming. Freshly expired in September and October, slumbering in the freezer until called.
Conclusion: I love the SLX, I even go hiking with it and still take 35mm along as well. I originally read about the Hasselblads and wanted one, but they are ridiculously expensive again. Then I saw the Rollei mentioned somewhere and found the first one online. There are no foam seals on the back. All solid hard plastic and metal, except for two foam mirror dampeners, which look like fresh from the factory. For me as a layman the cameras look like they were designed to be completely maintenance free. I guess I'll have to wait and see.
Next week I'll fly to Hong Kong with it. One SLX, two Minolta 35mm and digital system into one photo backpack as my carry on. Holy trinity of lenses each for the two film formats. The Olympus digital and a few lenses fit into the cracks left by the film equipment.