I bought a D5100 before a trip to Italy and was very happy with it.
It's light enough that I didn't notice the weight on a long strap over my shoulder/neck, less than 2 lbs with the 55-200mm kit lens. I wore the camera all day, easy to just grab and snap a shot instantly. An 18-200mm would make it about 2.5 lbs; I would have preferred this over swapping with the 18-55mm I brought, but it's a pricier lens.
I was somewhat disappointed with the PD auto-focus. A lot of pictures are a little soft, which I didn't notice until I got home. Mostly this was using Auto-Area AF with people in the foreground; the little AF red lights flashing in the viewfinder led me to think it was choosing well. I saw no camera/lens problem after doing some tests, so I think my expectations were too high or technique poor. I took photos before the trip with most every photo well focused, but I think the vacation scenes were probably much more complex. If I'd better understood the AF limits beforehand, I could have managed them--taken some extra photos, used center single-point AF more often, Live View when time permitted. I also have a lot of trouble focusing on black dogs running around; the D5100 AF is just not up to that.
I expect the D7000's superior AF module would do better; you might consider that, with the trade-offs of ~+6 oz to carry, price, and articulating screen.
Otherwise, image quality was great. Matrix metering almost always worked perfectly. Low-light or under-exposed images look great after Lightroom recovers the shadows. Sensor quality amazes me. VR on the lenses is like magic.
Switching settings on the D5100 for different situations is rather slow with lots of button presses for ISO, AF modes, AF area, exposure delay mode, HDR, etc. I took quite a few bad shots because I didn't have time to switch it all back or forgot something. The U1/U2 modes on the D7000 sound really nice.
The articulated screen was pretty useful--to get the camera above crowds, low to the ground, and into odd places where I could never position my eye. It's also handy for the occasional video.
I had a Canon S90 as a pocket/backup camera but only took a handful of pictures with it. I took a lot of pictures with the D5100 that the S90 would have been too slow turning on/focusing to get.
I brought a pocket-sized Sima tripod (3oz), which, surprisingly, can (just) hold the D5100 even with 55-200mm lens. This, along with the articulated screen, was useful for long-exposure night pictures from balconies. I was thinking it would be useful for portraits with a remote, but between crowds and little time in one place, there was no opportunity.
The D5100 was a good choice for me. Not too expensive, light enough to wear all day, lots of great results.