I recently made the switch from the D80 to the K5, so I'll chime in. For reference, my Nikon equipment was: D80 w/ grip, SB-600 flash, 24-120mm f/3.5-5.6 VR, 50mm f/1.4, Macro 60mm f/2.8.
- K5 is a cropped sensor, with a factor of 1.5. So, 50mm will have the field of view of 75mm. That places the DA* 55mm f/1.4 at about 82.5mm, making it one of the current portrait lenses for Pentax (others being DA 70mm Limited and FA 77mm Limited).
- The Pentax equivalent of the SB-800 is the AF540FGZ. Check out the posts on bounce flash overexposure with the K5. It does happen; although I've managed it well enough. FWIW, in similar circumstances, my D80 underexposed. (Side note, I'm curious to see if the Highlight Correction feature will work with RAW in Lightroom, since that would be a BIG help.)
- The K5 can shoot either the Pentax RAW or the Adobe DNG RAW format. I use DNG, which can be processed in Lightroom, Photoshop, and Photoshop Elements. The latest versions of those can also work with the Pentax RAW from the K5. (Side note: if you like the control point feature of Capture NX, Nik Software (Nikon's partner in NX) makes a plug-in for Lightroom and Photoshop that give the same functionallity.
- The D300s is really getting old. Feature wise, it's above the D7000 (barely), but image-quality wise, the D7000 is better. I consider the K5 and D7000 to be very comparable, with the exception that the K5 has in-body stabilization. The D7000 has a better autofocus system and, arguably, a better flash metering (Nikon probably has the best flash system in the business).
- Coming from the D80 w/ grip, I find the K5 w/ grip to be perfectly comfortable. The only gripe I have with the ergonomics is that the four-way buttons used to select the focus points also have other uses, and you have to use the OK button to toggle between them. They really should have made other dedicated buttons; trying to change the focus point and getting white balance selection instead is annoying.
- One of my main reason for switching to Pentax was their reasonably-priced DA* zooms (16-50mm f/2.8 and 50-135mm f/2.8). Nikon does not appear to have a desire to release similar lenses. They have the 17-55mm, but it is not worth the $1300 they want for it. The quality of the DA* zooms isn't quite as good as the top-of-the-line Nikon and Canon (24-70 f/2.8 and 70-200 f/2.8), but they are still very good and are less than half the price. Only real place where Pentax is lacking in lenses is for above 300mm, but I don't have the $$$ (correction...$$$$$$$) for those anyway.
- Whether the "stain problem" will be an issue for you really depends on you. Are you someone who is going to look for it and stress if you find it? In most real-world shots, you won't see the stains even if they are there (someone correct me if I'm wrong...but I believe that it shows up at f/16--f/22). I don't use those aperatures, since diffraction is already taking a heavy toll on the image quality. I have seen some posts regarding them showing up in macro shots (where cloning them out isn't really an option), but I haven't paid much attention. May need to, however, since I just got the 100mm WR macro for Christmas (wonderful lens, by the way).
- Should you consider the K5...definitely. Is it a no-brainer...no. The Nikon D7000 is, by all accounts and measurements, a very capable camera that can compete with the K5 in pretty much every specification (in-body stabilization being the notable exception).
In the end, I changed over to Pentax because I did not forsee Nikon releasing the kind of quality DX lenses that I wanted, and I was unwilling to pay the extremely high prices that they are asking for the top-of-the-line FX lenses.
The flash issue with the K5 is somewhat disappointing, and I am hoping for a firmware update (and/or for Lighroom to support Highlight Correction...fingers crossed). However, for me at least, the problem seems limited to low light (i.e., high ISO) and is typically correctable with flash compensation. (Note that under similar conditions, my Nikon tended to underexpose the scene...pick your poison I guess.) There have been only a couple scenes where the camera just completely failed to meter the flash correctly. Not sure why, but it overexposed every time (and I tried about a dozen shots). Changed positions and it worked fine. Something was obviously messing with the metering sensor with that specific scene. Bottom line is that if you only have one chance to get the shot, either dial in -1 to -1.5 EV in P-TTL mode (and push in post if needed) or use the flash's auto aperature mode.
