If you subbed to Rift before you can probably log right in at the moment even if your sub isn't active any more.
If you sub you'll get exp and favour boosts.
This is for a few days to celebrate Rift's half-birthday. In keeping with Trion's practice of doing everything faster than WoW they even have anniversaries faster.
I'm enjoying it so far. My one complaint would be a lack of information about server groups and LFD queues are still horribly long. Still kudos to Trion - I had had no intention of ever playing Rift again and now I'm quite tempted to sub for a month and play my mage.