I have been shying away from #regions for awhile now, but the deal clincher occurred after I read Clean Code (review coming soon). I have always thought that good code looks good. The book makes a great argument for this by describing how good code "reads like a story". You can read it top to bottom. It contains descriptive, well-purposed names and the methods appear (generally) in the order they are used. Additionally, a good class contains a single story. That is, it should have a single responsibility (SRP) - one reason to change.
As I see it, #regions:
1. Prevent me from reading the story. If I have to open #regions to inspect a class, it hinders my flow.
2. Prevent me from writing the story in the first place. For example, a class whose methods are grouped into #regions called public, private, and protected, does not make for a good story. It's a bad read because I can't follow the code from top to bottom. I end up bouncing around the code like a spider-monkey playing ping-pong. This makes it difficult to ascertain the purpose of a class that I wrote a long time ago.
3. Might indicate that I have more than one story. If I am grouping methods together based on the purpose of the methods, it may be an indicator that I am dealing with a class that needs to be split up.
4. Could possibly be nothing more than a bad comment. One attribute of a bad comment is that it is redundant in regards to the code that it is describing. #regions can be like that sometimes. Consider a #region named "Public Methods". I can look at method and clearly see it's accessibility. Why denote it twice?
5. Are unnecessary because I use ReSharper. It is trivial to navigate code using ReSharper. If you are a .NET developer and not using a tool like ReSharper or CodeRush, you might as well be coding with one thumb. With ReSharper I can use ALT+DOWN to scroll between the methods of a class or CTRL+F12 to jump directly to a method.
6. Are kinda crufty. They are there so the IDE can collapse/expand regions of code. They don't really do anything.
That's all I can think of right now, but these are enough reasons for me.