There are typically 1500+ in these, with only 50 people making it to round 2. In round 2, there are a little less but only 30 seats available. Round 3 is when the actual prize is awarded - but only to the top 3.
I have made it past Round 1 several times, it is actually pretty easy. As with most freerolls, there are a lot of sitters and all-inners. If you wade through the first hour, there's a good chance you can make it.
I have only made it past round 2 once though, which was this weekend. With only 3 seats available, I imagine it will be much tougher.