I have come from experience with using the DevExpress layoutcontrol and unfortunately theirs is far more matured than yours. Some improvements i strongly think you should make to allow this control to be a part of every Winforms app:
1) Intelligent auto-enabling of "DrawText": Input style controls (TextBox, Combobox, listbox, etc) should have DrawText enabled by default, while others (Label, Buttons, etc) should have it false by default.
2) Text for added controls should be sized better: At the moment, you can only choose from "proportional" or "fixed". The problem is that "Proportional" never looks good as it creates too much space for the label, and "fixed" forces me to have to make a decision on how big i want it, and if later on i add something that is larger, i now have to go though all the controls and update their size. A Better solution would be to have an "auto" option that behaves like "fixed" but sizes all on the labels in the same container also set to "auto" to the greatest of all of them.
3) Size the LayoutControl relevant to the control that it is containing: A Textbox generally has fixed height, so making the layoutControl larger than it just leaves unusual spacing. This is more pronounced the more controls you have being laid out. Moreso, this even allows the child controls to grow larger than its container, potentially having them overlapping other controls. This to me defeats the purpose of a dynamic layout. So far, the only way i have managed to achieve a consistent layout regardless of window/form size is to set the min/max size of the LayoutControlItem which is cumbersome.
4) Right-click context menu at design time: Take the DevEx one for a spin. Right click on a layout control and you can do things from hiding/showing text, fixing width and/or height, can multiselect and then click "Create Group". This alone would cover my first suggestion.
5) Add panels to the layout control: At the moment, once you add a panel to the layout control, you can no longer layout the panels contents as any control you try to adjust will try to snap to the LayoutControl. Sometimes your design requirements will be outside of what the LayoutControl can provide.
6) The "Customize" overlay is neat but...: I like how this allows me to focus entirely on laying out the "LayoutControlItems" however, sometimes you need to modify the properties of the LayoutControlItems to get the desired changes and as the properties window is no longer accessible (due to a modal popup i guess) this is fairly crippling. Considering that nearly all of the needed layout functions are accessible only through the properties (i.e setting min and max sizes to constrain proportions. See item 3 about this), it means i cannot use the Customize mode to its full potential. Perhaps taking advantage of your Properties editor to appear in the customize overlay would alleviate this problem?
If you are a winforms developer, chances are you are using the designer to lay things out and the LayoutControl is potentially a fantastic tool to augment this.