I played it for around a year, that for me was back in summer 2005, until the summer of the following year. I remember most of my friends playing it as well telling me that I happened to play the game at a moment when there seemed to be a considerable number of players simply quitting it, and that the best years for the game were 2003 to 2004, at which point it was quite popular. But its popularity dwindled fast, apparently. I myself never noticed the actual drop in the number of players, but I kept seeing chats about how "better" it was "just a few months ago".
I chose to play for the Terran Republic, I thought it was fun, especially sniping, or leading a group of buggies at enemy bases and proceeding to its invasion, thinking about that retrospectively it was great fun really, I liked it, but I had to quit due to lack of financial resources since I had other priorities back then and I only worked during the summer, but I did not regret trying it and paying for that for about a year. If there's a sequel I might be interested, but I will have to chose to play only one massively-multiplayer game at once, either Old Republic of PlanetSide 2... that could be a tough one for me.
In my book, and for the time I have played it, I wouldn't say that PlanetSide "failed", but I don't know what happened after summer 2006 when I had to quit, I never followed any news about it after that. And to this day, PlanetSide and Star Wars Galaxies have been the only two massively-multilplayer games I've ever played (I only played Galaxies for three months).
