What worked for me is not only setting the Canvas.enabled=true, but also turning the CanvasScaler.enabled=true.
design the panel and then disable the canvas and canvasscaler. When u want to show it you turn both on and the text is crisp., when you close the panel, disable both.

Pattern is -
OpenPanel()
myCanvas.enabled=true
myCanvaScaler.enabled=true

ClosePanel
myCanvaScaler.enabled=false
myCanvas.enabled=false

NOT SOLVED! Env: Unity 5.4.2!
When text without not pixel perfect:
1. width\height with .5f div
Or 2. paragraph alignment "center"

Curious. I'm wondering if this is related to a problem that appears to have regressed in current builds. Bring up the Scene and Game windows and put up some bright blue text on a black background. Do they look the same? What if you switch Color Space between Linear and Gamma in the Player Settings?

STILL NOT SOLVED in Unity 5.4.0f3, I'm struggling with the UI Text always getting blurry since the release of Unity 4.6, regardless of the Canvas Render Mode, and it can be quite hard to work around the problem when using a Canvas Scaler...

NOT SOLVED! Env: Unity 5.4.0f3!
When text without not pixel perfect:
- width\height with .5f div
- paragraph alignment "center"

UGUI text in buttons and directly on canvas all look fine in editor and game window until pressing play. When my game brings up the canvases the text is blurred in both the editor and the game window. Attempting to resize the canvas in the editor snaps the text into focus. Also, have a scroll view which if you scroll, it snaps the text there into focus as well.

Using Screen Space Overlay with Pixel Perfect checked. Canvas Scaler set to Scale With Screen Size.

I wish we could just bring Visual Studio's Win Forms Editor into Unity - so easy, so freaking easy!

NOT SOLVED!

How do you work around this when your rects are driven by a canvas scaler!!!!!