In terms of interactivity, I think the Otocinclus rank last among my fish. Everyone else is at least aware of your existence when you feed them, but since these guys aren't interested in fish food, there isn't any reason for them to pay attention to me. As "sucker-mouth" catfish they are also different to the other fish - they will often just hang on the glass at the back of the tank for extended periods of time, or on the petioles of an Echinodorus leaf.
I bought three of them, and as recently as Saturday I was able to find all three, but the last few days I can only find two of them. It's possible that I lost one, although it's hard to say for sure, given the density of hiding places in the back of the tank. As the fish most inclined to sit still, you'd think that they'd be the easiest to photograph. Unfortunately, they tend to sit at the back of the tank, obscured by either leaves of bubbles. They're actually almost impossible to photograph.