I just inadvertently played a 3DS. I have a friend at GameStop and he helps me find obscure titles when I need them. So I'm in there looking for a game and we got to talking and I asked how the 3DS was selling. He pulled out the store's system and hands it to me.
Here's what I think. Sorry if I'm repeating anything that's already been stated.
3D: I had to have the slider at a little less than halfway up the scale because the highest intensity just hurt my eyes. I guess I'm just not used to it. It's impressive and really gives a sense of depth but I found the game to look just as good in 2D mode. (I was playing that racing game.)
Graphics: Graphically, the 3DS is impressive. From what I've seen, it looks almost as good as the Wii. (Just my opinion; not fact, mind you.

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Controls:
1) Stylus- I like the design of it. Being able to adjust its size is great but its location on the system is a poor choice. I think most people are right-handed so maybe it should be on the right. Maybe whenever a redesign comes around they'll change that. However, it's only a minor problem that I can get over.
2) Control stick- A very, very good control stick. Its movements are smooth and it has adequate range. I wish it could move a little further but it still works great.
3) D-Pad- Uh... No. It has the same problem I have with the PSP's control stick. It feels tacked on and is in a poor position. I couldn't comfortably grip the system and play with the d-pad. It probably would've helped had I been sitting down but even then I don't know how easy it would be to use. It felt a little cheap too. Although, I'm sure most games will use the control stick so it isn't that big an issue.
Definitely not my favorite DS directional pad but I guess it'll do the job.
Build quality: The unit felt very sturdy but the outer casing only felt slightly tougher than a DS Lite but overall it felt all right.
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Overall:
I'll probably get one but not until the software library makes it worth it. It'll probably be a while before a price drop so it looks like I'll be paying its current price. The 3D effects are nice but shouldn't be the sole reason for a purchase. I definitely recommend trying out a system in-store or playing a friends before committing at this time. The software library looks okay for now but it might be worth waiting for the summer titles.
The 3DS feels less like a new game system and more like an improved DS... Which isn't a bad thing. I have some high hopes in Nintendo right now and I'm sure that this will be a winner.