I am thinking of getting the Nikon D3100/nikon j1.
How do you find it to use?
My girlfriend has the nikon J1 and I use to own the D5100, which is the version above the 3100 but similar to it. The J1 is a great little piece of kit, yet in manual its not the easiest of cameras to use as you have to go deep into menus to change the ISO unless you leave it on one of 3 automatic ISO modes.
The ISO performance on the J1 is pretty damn good, as is the vibration reduction which is sensor side, not lens. The lenses however can be pricey and to really get the most out of the camera it requires the F mount adapter, which is around £140. That enables all of the other nikon lenses to fit the J1, giving you some insane reach for very cost effective prices. Thats a MAJOR plus for wildlife photographers especially.
Downside is there is no viewfinder, the autofocus system takes some getting use to (has over 100 points!) and it can feel very odd to hold until you get use to it as the ergonomics seemed to be an after thought.
The D3100 is a nice starting DSLR which has vastly easier full manual controls when compared to the J1 system. Also the lens choices are easier to understand and you get better bokeh with this camera over the J1 due to the sensor size being bigger. Downsides to this camera compared to the J1 is mainly the autofocus system which isn't a patch on the CX format one, yet its good enough for everyday tasks. It definitely isn't a wildlife camera though, especially if you want to take photos of birds, although it CAN do this still, just means you need to lay out more cash on greater focal length lenses to get the same level of magnification the J1 system has.
D3100 has a crop factor of roughly x1.5 whilst the J1 is roughly x2.6. I'd say the ISO performance on the J1 is possibly better than the D3100 as it really surprised me, even at 3200 iso!
Definitely go to a shop and try both of them though before you remotely consider buying either of them as they are completely different systems with big strengths and weaknesses.