The 3DS is a good system, though I would gladly trade the 3D for higher resolution. I don't use the 3D much, and I feel the low-res screens are the main area where the system is showing its age.
Like ejamer said, the build quality was lacking, at least for the original model which felt like a cheap prototype. I haven't had any issues with my XL, so that model is a lot better and seems fine. The placement of the Start and Select buttons is still poor, but that's about the only negative I can say about it.
Software-wise, I enjoy both the game library and the apps. StreetPass was a fun novelty, and I also like to browse the Activity Log now and then. While the original DS probably has more good games overall, the 3DS may have more great games, so it's tough to say which is better. Though, the two systems kind of blend together for me since I didn't have a DS, so 3DS has been great in that it's also allowed me to experience the DS library. I think I do like 3DS more than Wii U though. I at least have more playtime on 3DS.
There are still plenty more 3DS games I plan to play and buy, so I'll still be playing the system for years to come. I may need to get a NEW 3DS at some point, simply because my current system will probably wear out from use. Though currently, it still works fine.
My current top ten would probably look like this, though it's definitely subject to change as I play more games:
1. Luigi's Mansion 2
2. The Legend of Zelda: Tri Force Heroes
3. Pokémon Super Mystery Dungeon
4. Fantasy Life
5. Fire Emblem Awakening
6. Puzzle & Dragons Z / Super Mario Bros. Edition
7. Dragon Quest VII
8. Super Mario 3D Land
9. Monster Hunter 4 Ultimate
10. Bravely Default
It's a much better system than I gave it credit for, I just shouldn't have been wasting my time with high profile titles I didn't like much like NSMB and Phantom Hourglass.
I wonder what would happen if you gave the Wii the same benefit...