GlassJaw
Hero
The best time I feel to play a mmporg is when its just starting up, there aren't any gold sellers spamming you, everything has to be earned as people are unwilling to sell things and there is a sense of wonder as most people are exploring for the first time, also groups at low level are easier to find. Although you may have to cope with laggy servers / crashing.
I think the best time to play an MMO is a month or so after it's released. MMO's ALWAYS have problems upon release, and giving it a month lets the community determine how bad those problems are. Hopefully, the devs will issue some patches to fix the major problems and you're good to go. Worst case scenario is that the game utterly sucks but hey, you didn't waste your time.