My journey has been a bit of fun over the past 5 years.
I've been from:
G10; 40D; D200
G10; 40D; 5D; D200
G10; 40D; 5D; D7000 (D200 sold shortly afterwards)
G10; 40D; 5D; D700 (D7000 sold shortly afterwards)
40D; D700; A7r (G10 and 5D sold shortly afterwards)
40D; D4; A7r (D700 sold shortly afterwards)
Started with a G10 new, and enjoyed myself. That is until I started browsing this forum and looking on ebay at photography gear: downhill from there.
The first used purchase was a 40D to "progress" my photography. Then because I was using F mount lenses a lot with photo finish systems for track and field athletics I got a D200 (as I had by this point started acquiring Nikon lenses).
At some point I stumbled across as 5D for a really good price so gave that a try to see what everyone was raving about (FF sensor, film like images). All I discovered was a slower body which had at least a stop, maybe two on the 40D in low light. At this stage I hadn't sold any of the previous bodies either.
Next up was a D7000 on the back of a lot of excitement about potential DR from the Exmor sensor. This prompted sale of the D200 as the D7000 did everything better. That body was good fun to use and in hindsight, not really outshone in some aspects by later bodies I've bought. It also introduced me to Nikon's AutoISO implementation which I think is the best out there (particularly in the newer bodies like the D4) and alleviated the idiotic positioning of the ISO button.
The only issue I had with the D7000 was fps and buffer depth. Options at that time in my budget were D300s, D700, D3, D3s or the brand spanking new D4. Unfortunately the pro bodies were out of my budget so following my experience with the 5D in comparison to the 40D (low sensitivity IQ mainly) I ended up with a D700 rather than a D300s. This ended up being a bigger body size jump from the D7000 than the 5D was over the 40D but the D7000 soon departed to bring me back to 3 DSLRs and the old forgotten about G10.
I was pretty happy with this setup for a while until the Sony A7 and A7r were announced. I treated myself to a new A7r (birthday present) for an experiment and was blown away by the resolution and IQ of the sensor. The 5D, G10 and most of my Canon lenses (which overlapped with F mount lenses in my cabinet) were sold off to offset some of the cost of the Sony. This was a fun camera and became my goto walkaround body with an adapted F mount 35 Art. The D700 was consigned to sport duties while the 40D was left with my trusty old Sigma 18-50 because that combo was way better IMO than I could get on the used market.
After a few motorsport events last year I got the itch again to update my main sport body - the D700. I wanted something with a bit more resolution, quicker fps potential and deeper buffer mainly. As the D700 was at this stage always been used gripped, size was not a great concern. The D810 had been released by this time and with a couple of the quirks of the D800/E sorted and having gotten accustomed to the file sizes from the A7r I was interested in a gripped D810.
For whatever reason though D4 bodies were going for really competitive prices on ebay. I would have ideally liked something in the order of 20-24mp but 16 was a good enough upgrade over 12mp. When I priced up a grip on the D810 being higher than a used low mileage D4, saw that a gripped D810 was going to be bigger and heavier than a D4 and remembered that I really did want the potential for a quicker frame rate than the D700, the D810 fell out of contention, particularly after I found a really, really good priced D4. That was October last year. The D700 went pretty soon after as the D4 proved itself to be better for me in pretty much every way. Particularly after I found the option to set the record button near the shutter release to change ISO - no more awkward fiddling trying to change ISO.
The D4 eventually proved to be so much fun for me to use that the A7r started sitting around gathering dust. I wasn't missing resolution all that much, the DR from the sensor was good enough for my uses and its low light IQ was a couple of stops better in my use. Coupled with a much nicer AutoISO and AF (which in my use was much quicker and more reliable than focus peeking and magnification) I eventually decided that the cash I could get for the A7r would be better invested in glass, so now I'm back to the D4 which gets used for everything and the unused 40D (which will probably never be sold).
What's next? Maybe a D750 sometime next year for a lighter body. Or maybe I'll manage to settle with this gear until prices of the rumoured D5 drop to a sensible level.