Bigger pixels.

Whether it is worth it again depends on what you want from it. Will the top-of-the-line model be worth it over the base model? It depends entirely on what you want.

Using Canon full frame as an example, the top-of-the-line newest 1Dxwhatever is certainly better at many (most) things than the legacy 6D, but is it better for you? If you shoot sports or birds, then it probably is. You travel a lot, the 6D is likely better and on account of its light weight and excellent low-light capabilities is perhaps the best Canon DSLR of all for travel.

The Nikon Df is inferior in every specification, not to mention more expensive than the D810, but that doesn't stop many photographers from really enjoying its retro control layout and dials that can be preset with the camera shut off (I love mine and converted its focusing screen to an old-style split image). Even the newest D850 would still be "less fun" for me to use than the Df, despite its many advantages.

Leica is probably the best example of an over $5000 camera. Nobody really "needs" a Leica rangefinder, and while the image quality is outstanding, Leica's sensors are no better than Sony's or Canon's (no worse either in image quality, though deficient if comparing spec sheets). Leica's lenses are all outstanding, and outstandingly expensive, but as one example Canon's newish 35mm f/1.4 L II is every bit as good as Leica's 35mm f/1.4 Summilux at roughly 1/3 the price (and twice the weight and three times the bulk). Of course, Nikon, Sony, Canon and the like don't offer a proper rangefinder, and until they do photographers who like rangefinders will keep buying Leicas.

If you mean man cal focus, then my answer is sort of. I have a Nikon Df that I had modified with an old-style split-image and microprism focusing screen from a Nikon FM3a and use it exclusively with vintage manual focus lenses. It works just like an old FE or F3 would, only with the benefits of digital capture and focus assist (the AF system still works even if the lens is disconnected, and the arrows help in difficult situations).

If you mean medium format, not yet, though have used MF for film and am eager to move to MF for digital when prices come down a bit more. Pentax 645 and Leica S systems are both very appealing to me.

I've bought a few over $5000 cameras, and a few over $5000 lenses. I blink a lot more than twice when making such a purchase, but its no different than buying a very high end musical instrument, a fine wristwatch or drinking expensive wine. If doing so either helps in your work or play or just makes you happy, and you can afford it, then go ahead, you only live once.

As I got older, I moved to full frame Leica instead of crop sensor Fuji. Small, light, and with all of the advantages of full frame.

Who needs it?

Well, people who want more control over depth of field, people who want better (rather than just more pixels), people who want more (and still better) pixels, people who want to use a larger and brighter viewfinder (DSLR only), people who want the best lenses, and to use all that those lenses have to offer.

The simple fact is, with the same pixel count and equivalent technology larger pixels let in more light and are better than smaller ones giving a 24MP full-frame camera with similar technology higher dynamic range, better sensitivity and just better handling of light.

Likewise given the same size pixels and same technology, image quality per pixel will be identical, but FF will just give you almost twice as many of those identical pixels.

Crop has advantages too, like smaller files, smaller cameras and lenses, and allows an equivalent narrow field of view with comparatively shorter (and therefore even smaller and lighter) lenses. FF has the same advantage as you move to wide angles, not requiring as radically short of a focal length to get the same field of view that would require an extremely short focal length on crop.

Depth of field, to me, is the big one. I shoot mostly with a fast 50mm lens and have a lot of control over depth of field through the use of shallow aperture. A 50mm lens at f/2 has far narrower depth of field (and more background blur) than a 35mm lens at the same aperture, which is what one would use for crop, which itself has much narrower depth of field than a 25mm lens at f/2 that you would need for micro 4/3.

If you plan to use a slow kit-zoom, then it probably makes very little difference though.

I have a very lightweight, sturdy (and unfortunately expensive) Manfrotto tripod, but even not reversed, tripods are still annoying enough that it usually stays home. Fast lenses and/or high ISO can usually give fast enough shutter speeds to avoid camera shake even in the dark. My dislike for tripods is one of the primary motivations for my love of fast primes and rangefinder cameras which (like mirrorless cameras) do not have a mirror flapping up and down.

Rigid. Lightweight. Affordable.

Pick any two.

Dollars spent, hmmmmmm


Leica, Canon, Nikon, Sony (tried and abandoned mirrorless).

When shooting digital its probably Leica 70% and Df 30%, with about the same ratios when shooting film with a Leica M5 and Nikon F2A. do mostly street, travel and event photography, and almost everything with either a 24 (Nikon), 28 (Leica) or 50mm (both) lens. A lot of available darkness stuff.

Nikon Df and Leica M-D. Sadly on the Leica side, there are no lower-cost alternatives that have a true optical rangefinder and a digital sensor. At least used ones are far more reasonable than new ones and they are quite durable.

JPEG?

My primary camera shoots RAW only, no HDR, no scene modes, nothing at all.

Put a cheap and fast prime lens (35/1.8 G or 50/1.8 D) on your camera and the go out and shoot. Walk around, get low, shoot in the dark and play with different aperture settings. Zoom with your feet.

Feet are the ONLY zoom I use.

I don't have my Canon kit anymore, but when I did I used the 35/1.4 L II for about 30% and the 50/1.2L for about 67% and the remaining 3% spread over 24/1.4L, 100/2.8 L Macro and 135/2 L.

That 35 and 50 were very special lenses.

Its a real great one. I wish Nikon had one as good.