I like the Mark IV with TMR reticle as it hits the sweet spot in terms of size, quality, zoom range and clarity for field use.
If you start with the assumption that a field scope needs a reticle to be able to measure targets/misses/etc...you already have a pretty small set of scopes to choose from. Next you need to consider the zoom range - and 12-40x60mm seems to be the "useful" range - the 12x is wide enough to actually find targets when transitioning from a pair of 8x or 10x binoculars. Something like a 20-60x would be useless as it would be like trying to spot targets through a soda straw.
Moving to the high end, at 40x, the scope has enough power to measure and spot hits over 1000 yards away, lets you mil targets and misses with the TMR reticle, and has enough power to determine mirage so you can make the right wind call to make a hit.
It does all of this in a pretty compact, field tough package. Marry it with a strong carbon tripod and Manfrotto geared head and you can aim it precisely in the field. There is a reason why the military snipers use this scope.
Is it perfect? It could use a bit more eye relief perhaps, and something like a 20-60x65mm Swarovski probably has a bit more clarity, but the Swaro at 20x is not that useful in the field and most of their models have no reticle. Having looked through quite a bit of Bushnell and Vortex glass, I would certainly put the Leupold well above those.
I took the 7 day precision rifle course at Gunsite and we used Mark IV 12-40x60mm with TMR exclusively out to 1000 yards. Each of us spent about half of the course behind the spotting scope, and all of the targets were hand milled by the spotter. The Mark IV performed beautifully. Also the instructors there include some of the top shooters in the country - and I'm guessing if they could find a better scope for field use they would already be using it.