For me, anything smaller than 5x7 is "toy format". That said, I have an RB67 and a Rolleiflex. The RB is reserved mostly for studio shooting when I want to do color, and the Rollei is for snap-shooting on the street. It's really a matter of horses for courses - when I'm traveling, especially someplace I've not been before, I take my Contax G2 outfit. Some day I will actually take my 14x17 camera on the road with me, but that will wait for a time when air travel either becomes reasonable again, or for a destination I can drive to.
I like the square 6x6cm format, so that draws me to use MF sometimes. (TLR user here)
I use 35mm if I NEED autofocus (such as photographing kids in existing light, etc..) and 1.4/1.8 lenses. I don't use it as much as other things because I'm kinda spoiled by the image quality of MF and LF. That said, the grittiness of 35mm is sometimes just what you want too.
I'll use the TLR for kids indoors IFF I am using the big studio flash where I can set the aperture and not have to focus constantly. I'll take the TLR for walks sometime with B&W film instead of the DSLR if I don't feel the color quality will do anything exciting in color digital. Lots of non-tripod applications for MF. TMY2 film really makes a high quality B&W negative in MF. I've made some nice 15" square prints on 16x20 paper. I don't decide purely on the basis of weight but my Yashica TLR seems lighter than my F4s+lens. A TLR is also deceptively cute and friendly if you are photographing people. Not the most versatile technical choice, but people love them.
Then for the next step up in image quality, I'll put a couple film holders (in antistatic bags) in my pocket and drag around my 4x5 speed graphic and maybe tiltall tripod. Stunning 16x20 B&W images. Every shot can be processed different if I want. Far surpasses the MF, the DSLR, etc... Not practical if you need to be quick. More DOF and lens options than the TLR. Can even do instant fujiroid pictures.
B&W MF square is my mainstay and 4x5 when the situation permits. 35mm for family parties so I don't have to make prints for everyone. The family party stuff is switching to dare I say "digital" so I don't have to make a trip to the drugstore - just burn a cd or email and they are on their own.
Tomorrow morning it's into the darkroom with some new negatives eagerly awaiting to be printed. Ilford Delta400 in 120 and Ilford HP5 in 4x5.
Like some of the others here, I like the 6x6 square format, too, and I like the compact, self-enclosed nature of TLRs, so I use a Rolleiflex most of the time. I probably alternate about 50/50 between MF [almost all with the Rollei, although I do own other MF cameras], and 35mm, where I use rangefinders and compacts most of the time, with the occasional SLR.
I find a TLR suits my way of working; it's compact, the image quality is more than adequate for me, it fits in a small bag and isn't any larger or less convenient than a 35mm SLR. I also like waist-level finders, so that suits me, too. I agree with everything jp498 says about the friendly nature of the TLR when shooting people.
I've experimented a bit recently with 5"x4", but, so far, I haven't found it offering me much that I can't get from my Rollei in terms of image quality [I'm not set up to make big enlargements].
I use my Pentax 645N the way many use 35mm. It's just a little heavier, but I can still carry it out in the woods a decent distance. I tend to use 35mm if I don't know what I'll find or if I'm shooting mainly snapshots. The 645 goes if I have a reasonable suspicion of finding something I'll want to enlarge. If I know I'll find something on an easy trail or close to a parking lot, I'll take the 4x5 (it's a monorail). I just picked up a speed graphic, so I may take that a little further into the woods.
Since I also have a currently 18 pound baby to haul around, that gets balanced against the camera weight. I'm looking for off-road strollers on craigslist.
The Hasselblad is my main camera, then 4x5. 35mm is not used much and when it is used it is for photographs on the fly.
Wow. Great question and awesome answers.
Seems like there's a whole lot of people who're into Medium Format who also get into Large Format as well - I've yet to try that out (though I should spend a year or two figuring out the cameras I already have), though the ground glass experience of a real good modular LF camera sounds tempting, I should still figure out how to exploit the different DOF and resolution aspects of the 645 and 6X6 format.
I just got into photography about two years ago, before I thought of photographers as tourists and annoying snapshooters and not as artists like illustrators and painters - the ones I looked up to as I love drawing the most to begin with, all in all I'm not too worried about always having a great quality image, as photoshop and drawing skills can often make use of those interesting, yet technically lacking shots.
Because of all that, and that I first learned about developing photos by B&W darkroom processing, I've yet to try out Velvia and color negatives processed from the store, and seeing as I just got access to a scanner, I guess I should leave my comfort zone of what I'm used to and try that as well.
Considering all the answers here, there still seems like there's a lot of compromises made for camera to film size ratio here, with the TLR's and all - seeing as I've to deal with something as "small" as a 645 system I guess I should just start getting used to stripping down the camera, only carrying one or two lenses, using the manual winder, maybe even the waist level finder and all that.
I carry a camera with me everyday, usually three at one time. For daily carry those cameras are 35mm and that is because of the convenience factor. I tried carrying my 645 and the 67 (not at the same time) but they were just a little to inconvenient to do so on a daily basis. Now the only time I carry the MF cameras is when I have a specific idea for a shot and I want the ability to enlarge beyond the 8x10 limitation of 35mm. I must say tho that if I load the 35mm with a slower speed film like 25, 50 or even 100 ASA, that I can usually enlarge beyond what is normally considered the limit for that format with out a lot of grain.
I've been moving away from 35mm format film (though digital is another matter: I'm moving towards full-frame 35mm format). These days I tend to use 5x4 for landscape and architectural photography and am finding my new (secondhand) Mamiya 645 Pro TL (with metered prism viewfinder) my preference for macrophotography - though I still prefer large format macrophotography with stationery objects. The Mamiya 645s are heavy when one is used to conventional 35mm cameras, but lightweight compared to large format equipment. Unless you need to do close-up portraiture or macrophotography, I would suggest you look at the Mamiya 7 (6x7cm format rangefinder cameras) series cameras. I know of at least one professional landscape photographer in London who uses a Mamiya 7 for ALL his landscape work, on the grounds that it is more portable.