Harmony of Despair I can say for sure that it's still quite popular. Been with this game since the beginning, and other than a few slow downs of players here and there, new batches of players and regulars pop up and keep the game going. The last I played (last month) had a good amount of players running.
Almost always you can find others playing the game to run with you, and there's also a few chat rooms who have regulars who play the game often (I'm part of one, and was hosting another for a time). I would be playing it right now actually, but I'm keeping off the PS3 for now due to some issues going on.
If you're really into it, you'll find the game has more to offer than what it tells you, such as the various tricks with each character and glitches for getting around. I feel I learn more and more every time I play it, and it's quick so its something you can pick up for a time then put it down whenever you feel like it.
As the others said, you can play it solo, but you'll have more fun playing the game with others.