I have three tanks, which all had gravel at one point. I've recently switched two of them over to different products. Now, one has sand (5g), one has eco-complete (20g), and one still has gravel (10g).
I like the eco-complete look first and foremost, and I expect that my plants will thank me later. They already seem to be positively responding to the new substrate.
Sand is quickly becoming my second favorite. I like the look better than gravel, and it should be much easier to clean than gravel (waste just sits on top).
That's not to say that gravel doesn't have its place, however. It would clearly be a better option than sand for undergravel filters (which don't seem to be used much anymore). I also think that gravel may hold a colony of bacteria better than sand only because waste goes into the gravel substrate rather than sitting on top of sand.
That said, I won't ever use gravel in a planted tank again, as I've had very little success keeping plants in that substrate. I'll only use eco-complete or another specialized plant substrate like Fluval Stratum for planted tanks from this point forward. That may cause pain in the wallet with 55+g tanks, but I'll cross that bridge when I come to it.