i've had my Nikon D3100 for a few weeks now and absolutely love it. i normally dont write reviews but i love this thing so much i had to take the time to write one. first off, this is my first dslr camera, so i'm new to this type of photography. all i can say about my Nikon D3100 is wow, it's an amazing camera. i've noticed some reviewers here saying "buy a d90 or d7000 to start", but i completely disagree. if you're new to photography, this camera is all you'll need for a long time.
i'm not much into video so i wont waste time here talking about the video options on the camera. i've taken a few and thought they were cool, the focus was good, the colors natural. having only a mono speaker the sound wasnt very good. but this is a camera, not a camcorder. and for photos Nikon D3100 rocks. though i'm new to photography i'm as ocd as it gets, and i've spent many many days and now weeks reading about how to take great pictures and use the settings on the camera. and i've gone out and put that knowledge to use.
i'm very very pleased with the image quality of this camera. in low contrast environments the autofocus will hunt a bit, but it's not that annoying. in dynamic area mode you can manually select your focus point, which is really nice when trying to get cool depth of field shots. the kit lense is good for general photos, but for really creative shots get a 50mm prime lense at 1.8 or 1.4 aperture. that's first on my accessory list! i've shot in both shutter and aperture priority mode, and had very good results so far. full manual is more challenging, but i cant blame the camera, just my feeble skills.
i've noticed the kit lense is sharpest at about f8, so i try to stay close to that, given adequate lighting. the image stays sharp from 18-55mm in the proper f range (8-13, in my opinion), then softens up a bit at the edges when you stop up or down. i've experimented with the iso alot and gotten mostly good photos. from iso 100-800 the camera is excellent, with no noticeable noise. at iso 1600 noise does begin to show, but for smaller cropped photos like 5x7 or 4x6 it's very usable. at iso 3200 noise is high and fine details are degraded significantly. but most users wont be using high iso so that really doesnt matter. and from comparison shots online the d3100 seems to offer very commendable high iso performance. it's not a pro camera, so dont compare it to those models.
to address some complaints that you have to buy only af-s lenses with an internal focus motor in the lense - yes you pay more for af-s glass, but the af-s glass (or G series)is superior do the af glass, with two more aperture blades, offering a better bokah and softer depth of field effects. i've seen that first hand as my friend shoots with a d90 and has both the 50mmD AF lense, and the 50mmG lense, both at 1.4. So though you'll pay more for glass, it's better glass, so it's a fair tradeoff.
i've used the guide mode some and it's very helpful, especially if you dont do your homework and just jump right into shooting. i wouldnt consider the guide mode a must have, or a deal breaker, it's just a useful tool. my biggest complaint with this camera is it's lcd screen. it's low res and not very good at representing waht the actual photo you took looks like. i dont use live view so it's not a big deal to me. but i do get annoyed when i think a photo looks good/focused/properly exposed on the lcd, only to find it's not when i blow it up using the viewnx2 software provided. oh, this software is really nice for a beginner like me. i was easily able to go into a photo (i shoot in raw always) and start adjusting it, and converting it to jpeg when i was done. sometimes my adjustmetns are for the better, sometimes not. but that's user error on my part, not problems with the software. other than the low res lcd, i have no complaints.
i've tried continuous shooting and the 3fps speed was fast enough for my needs. i've found color to be very neutral and natural looking, slightly cool. white balance in auto has been very good. no orange looking pics when taking pics inside with regular bulbs lighting the room. oddly though when i took pics in my own home, where i use all 6500k cfl lights, the white balance was a bit too white. being a beginner i cant really explain, but i played with the white balance settings and now it looks much better.
the ergonomics of the body are great. this is a very very light dslr camera with the kit lense. carrying it around all day at the zoo was painless. the grip feels good in my hand, and i like the rough looking texture of the plastic body. all in all this has been an amazing camera so far for me. the pictures are amazing, displaying sickening leveles of detail (like facial pores! ;)). it's been very easy to use and my experience so far has been excellent. i cant say it enough how much i love this camera, and dslr photography. this camera makes entering the dslr world very easy. and once you see the amazing photos this camera can take, you'll never think about your point and shoot again (i know i havent!). so dont spend too much time obsessing over specs and such, just buy this camera, you wont be dissappointed.