I bought a 2 body, 4 lens, 2 back, SQ-A and SQ-Ai based system from a good friend who migrated to digital acquisition. I'd been using Mamiya C series TLR's for about 20 years, and though I didn't NEED to change to an SLR system, I was hooked as soon as I tried the SQ-Ai. I've since greatly expanded my system.
The control layout is superb, film advance crank on the right, shutter speed dial on the left, shutter release on the bottom right front. The film speed is set on a dial on the interchangeable film back. This is a great feature if you're shooting with different film stocks in multiple backs as you don't have to worry about resetting the film speed dial with every back-swap.
The SQ-Ai body cradles perfectly in my hands when shooting with the waist level finder. When handholding with a prism finder, the Speed Grip S is pretty much a must. The Speed Grip S interfaces with the wind mechanism on the right (the crank must be removed), and provides a flash hot shoe on top of the grip, the sync connection made via the base of the camera.
There is an excellent selection of viewfinders available in both metered and non-metered varieties. As I write this, I have 1 SQ-Ai body and 2 SQ-A bodies (the previous generation). I have an AE prism finder, a Metered Prism Finder S, and 3 waist-level finders in my system. I like waist level finders when working on a tripod, especially when shooting headshots. The AE finder is handy when street shooting as it provides aperture priority automation as well as full manual control.
Speaking of automation, I'm one of those who tends to rely more on handheld meters than the in-camera variety. With the Bronica, the meter is in the viewfinder, not the body, so if you change to the wonderful folding waist level finder, you're going to need a handheld.
The SQ-A and SQ-Ai bodies are very similar, the chief differences being electronic. The SQ-Ai adds TTL flash capability and shutter speeds timed out to 16 seconds versus 8 seconds with the SQ-A. Since the basic body casting remained the same, the Bronica engineers had to find a way to get in the additional electronics needed for the TTL flash system and maintain 100% backwards compatibility with other system accessories.
The solution to their engineering problem was the battery compartment. The SQ-A uses a PX-28 size battery, the SQ-Ai uses 4 76 size cells in a special holder. HINT: Turn off your camera meter and lock the shutter release button before putting the camera in your case. Fail to do so, and you might find yourself with a dead camera as it's very easy to accidentally activate the meter and drain the batteries.
Speaking of being battery dependant, the SQ system uses electronically controlled Seiko shutters in each lens. The shutter speed control circuits are in the body and interface to the lens via gold plated contacts at the lens mount. I've tested all 5 of my lenses on each of my 3 bodies with my DCA1 shutter tester, and they are all perfectly on speed. Not bad for gear that was purchased used from various sources.
A word of caution is in order: Your system may start acting up as the battery level gets low even though the battery check says it's OK. One of my SQ-A bodies won't sync flash when the battery level gets down low. I found that out the hard way, at a wedding job. Fortunately, I always carry plenty of spare batteries, but I didn't figure out the problem until after the job, I just changed over and shot with my backup body.
I can't say enough good things about the lenses. The 150mm PS lens is killer for headshots, but you may want to use the S-18 extension tube for getting in real close. The 150mm is my favorite lens, but I tend to shoot more with the 50mm (about the same angle of view as a 28mm on a 35mm camera), especially for wedding candids. There is a full line of lenses from 30mm full frame fisheye to 500mm telephoto. Most common are the 50, 80, and 150 lenses, though some prefer the 180 to the 150.
Backs are available for 120, 220, 35mm, and Polaroid films. There are 6X6 and 645 versions for 120 and 220, and 24x36 and 24x54 backs for 35mm film. I'm not a big fan of 220, so I've invested in 8 120 backs, an extra 120 insert, a 24x36 35mm back, and a Polaroid back.
OK, this is getting long, I'll cut it short. Let me close with saying that this is the first medium format SLR system that could get me away from my beloved Mamiya C series TLR's. To be honest, I held onto the Mamiya gear for quite a while after buying the Bronicas, but it's all sold now.