I've been playing with the Windows 7 Beta1 since they came out. So far, I've been really impressed with what I'm seeing. I've installed quite a bit and have put a full development environment on it including Visual Studio 2008 and SQL Server 2008.
So far, the only real thing I've seen is a small bug in the LINQ to SQL and LINQ to Entity Framework design surfaces. In these, if you move the mouse over one of the entities on the designer, the entity may dissappear.
There's a quick work-around for this issue. Simply move the vertical scroll bar down. Once you do that, the entities will remain visible on the design surface. Rest asured, the development team is aware of this glitch and I'm sure there will be a fix for it by the time that the bits ship.