3D Logic is a perfect Flash puzzle game: it's simple in both concept and controls, no one level takes a particularly long time (although individual levels can be quite difficult), but overall it's engaging enough that you enjoy the experience.
The basic concept behind 3D Logic is quite straightforward. You're presented with a cube (or, more precisely, the three near faces of a cube), initially 3 by 3 by 3, but moving up to 6 by 6 by 6 at the end, of which some squares have been marked with a color. There are precisely two squares of each color, and your goal is to connect each pair of squares of the same color with a continuous path. Some squares are also blacked out so your path cannot pass through them.
The initial puzzles are quite easy, but they rapidly become more complicated. Though you soon develop a range of tricks to deal with some common situations, the puzzle-solving procedure (at least for me) involves a lot of trial and error, rather than pure deduction. So sometimes you will get lucky and stumble across the correct solution early on, while sometimes you can get yourself stuck in an unproductive corner for a while. In this case I highly recommend restarting the whole thing so you can start from a fresh slate and hopefully avoid falling into the same incorrect pattern as you did previously. The comments agree that levels 16 and 23 are the most difficult, and I think this is true -- the difficulty definitely increases at the beginning, but it kind of reaches a plateau near the end, so if you're a little frustrated, don't go too crazy.
The interface is quite simple, but effective. The one frustrating thing is that you can't clear a square once you've drawn a path there, so the only way to remove an incorrect guess (other than painting it over with another color) is to reset the whole puzzle, which is kind of annoying. There are a few small sound effects which are nice, but that's about it. Also unfortunately, you can't go back to a level once you've completed it. Note, also, that this game doesn't really need to be 3D -- since you only ever see the three near faces of the cube, you could just as easily map it onto a flat 2D surface without changing the substance of the puzzle at all. But the 3D cube does look nice.
Anyway, overall 3D Logic is an entertaining and challenging puzzle that will keep you occupied for an hour or so, depending on just how good you are at solving these puzzles.