Let me start by saying these are both awesome cameras. I use the Nikon at my place of employment and I use my Kx for my own freelance work. I've only used both for a short time since they are both new. I upgraded my system about the same time my job upgraded theirs.
My experiences today using the d90 and comparing it to my kx.
Well I shot in a dim atmosphere with a slow shutter. I don't have an SR lens for the Nikon so all the pictures came out blurry except the ones i took with the flash. My kx i could have used the built in SR.
Iso kx goes up to 12800 with very usable images I believe the Nikon only 6400.
The white balance on both cameras is pretty spot on but also a little warm on both. When I manually set the white balance on the D90 i have take a picture each time and then check it to see if its what I like. There is not a whole lot of tweaking I can for each factory preset wB. but it does have some and also has custome white balance and kelvin scale.
On The kx all i have to do is take a picture hit the WB button and it will pull up the last photo i took. I can then adjust the WB of that photo and even take out any colors by moving the dial to take out any color casts then every photo after that will be WB according to the sampled photo. This and this alone makes the KX a bigger winner compared toe the d90 for me.
To check the time i have to go into a menu system and look for it on the Nikon for the kx all i do is hit the info button. It may not be a big deal but I check the time regularly for breaks, clocking in and out etc.
Okay menus about the same. One goes vertical (Nikon) the other goes Horizontal (Pentax).
Size well the grip is better on the Kx and a bit longer too. with the Nikon I have about an inch of palm that hangs over the bottom of the body. A problem i never had with the Nikon d70.
I've been shooting with both Pentax and Nikon for a long time. Its funny how the Nikons are starting to look so much like the Pentax's from the back with an OK button and arrow dial that looks so much like the one on my ist DS . Even the new sb900 is having more and manual controls like the 540fgz flash.
They both have A-af, S-af, and C-af. They both have info on the back screen but the Nikon has top LCD panel which I really don't miss with the Kx cause its just more convenient to look at the back screen for the Info.
They both have live view and movie mode but the kx has the built in SR which helps a lot
They both have the AF/EL button. My favorite button on both cameras. Buttons feels the same on both cameras. Both have info buttons too.
The second wheel is not a factor either since I can't turn then both at the same time and with the Pentax all i have to do is touch the EV button to change the other. Pentax for me it is easier to control this way.
Build wise they feel about the same even use the same SD card door and plastic body. Both pale in comparison to the hard bodies of the Pentax ist DS and the Nikon d70. Those cameras look and feel as if they could take a fall and a beating. Both the d90 and the kx do not.
flash sync is about a draw Pentax syncs at 180 Nikon at 200. But Pentax has HSS sync which isn't available on the Nikon,,,at least I couldn't find it and it may require the new sb900 flash if it has it, as we are still sing the sb 800. This is a big Winner for the kx side as well for my own needs and uses. UPDATE>>>The Nikon also has hi speed sync.
Auto focus is pretty close with the Nikon winning this one. Very fast and very accurate. My kx is fast as well but its still using screw drive lenses.
There is no shooting delay on either camera with the Nikon jumping the gun a bit and i sometimes wind up firing two shots in stead of one. I definitely give this to the Nikon. Very good AF system...
Skin tones are good with both cameras and saturation as well. I would say both cameras are winner but for me there are things I miss on the d90 that are on my kx. I can definately give up a little speed for all the features the kx has. Including stuff like the filters and also the ease of changing jpeg size. d90 works on a L M S basic medium and fine scale the kx tells you 12, 10, 6, 4 meg and then you select the (*) (**) (***) for quality.
other things Nikon allows tethering Kx does not.
Nikon uses Lithium kx uses AA lithium or Nickle batts either way its a toss up here since amperage is about the same. Both last about the same as well. There are some new batteries in town called Zinc rechargeable that are suppose to be perfect for the kx. The voltage is a little higher than Nickel batteries but they are more comparable to Lithium batteries.
both have world time now, Pentax had it a long time ago.
Both have zoom review using the wheel again something that Pentax had long before the Nikon as my ist DS had it and my Nikon d70 did not.
Both have the on of switch around the trigger button but Pentax has the little blue light which I find really cool and distinguishable Nikon has a white painted line in the identical place and looks identical to the lit up one.
Forgot about the AF points. I miss them then I don't. Sometimes they interfere with what you see but I do like them but I don't miss them. But i'll give it to Nikon since I can tell where I'm focusing. But the 5 area Auto focus is more helpful than Nikon's full 11 point auto select. they both have 11 points.
The vga screen on the nikon can be misleading just like the screen on the canon Elf makes everything look better. Unlike my cheap samsung d53 p&s which the back screen looks like crap but when downloaded the images look incredibly pleasing. My sister has an Elf and all her pictures look amazing but when you download then they don't look so amazing.
It really isn't what you see is what you get. the screen makes everything look better and more dynamic almost a bit cartoony, but in the end its hard to know the real end product until you download and see it on a workable monitor and print them out. didn't find this issue on the kx. Even though a smaller screen not by much i found the images better represented and authentic. And the kx monitor can be color calibrated.
I've never liked Nikon's rendering in grass situations because it always exposes very cold.. Pentax always maintained a warmer feel in these situations.
As for auto iso both cameras have it. both have a sensitivity mode as well. Kx goes up to 12800 and Nikon upto 6400. Kx so far has the best noise control for high iso. It is very comparable to a full frame camera...infact in some tests the full frame only had a one to two stop advantage.