We just announced the latest MySQL Workbench 5.1 release, 5.1.16. This release was labeled GA, which means that everybody using WB 5.0 or an earlier version of WB 5.1 is recommended update to WB 5.1.16.
We are now putting our main focus on the WB 5.1 branch and will continue to provide rapid update releases during the next months. Please report any hidden issues you may still find on our bug-tracker bugs.mysql.com so we can take care of those quickly.
After the dust settles we will also continue to work on WB 5.2 in parallel, to give you query & administrative functionality within a single, powerful but easy to use tool.
Happy schema designing with MySQL Workbench 5.1!