I too have used ERStudio for years now. Good tool. It can get expensive, especially if you get cross-platform licenses. If you are sql server only they do (or used to) offer a much cheaper sql server only license.
Never having used a modelling tool, you may want to stick with visio though. ERStudio has a LOT of 'flexibility'. 🙂
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service