if invited definately go. Talent level can rarely greatly at open tryout camps. Many camps the talent level can to be kind embarrassing. you might see 30 to 300 players.
It depends alot on how good of job the scout running it is at attracting talent and does he know how to properly run a tryout camp
2005 tiger tryout camp result
http://detroit.tigers.mlb.com/NASApp/mlb/det/fan_forum/tryouts.jsp