Here's a summary of the types of properties you can specify on custom controls that work well with IntelliSense and validation:

• Attributes: 
   
  • Simple datatypes (strings, enums, etc.)
    <tag runat="server" simpleProp="simpleValue"/>
     
  • Any datatype with a [TypeConverter] that can convert 1) from a string to the desired type, and 2) from the type to InstanceDescriptor. 
    <tag runat="server" convertibleProp="convertibleValue"/>
       
• Nested tags when the [ParseChildren(false)] attribute is on the class or no [ParseChildren] attribute exsits:
   
  • Any literal text or control; all content is addded to the custom control's Control collection
    <tag runat="server">
        Literal and <div runat="server">nested</div> content
    </tag>
      
• Nested tags when the [ParseChildren(true)] attribute is on the class and [PersistenceMode(PersistenceMode.Attribute)] is not specified on the property:
   
  • Simple datatypes and any datatype with a [TypeConverter], as above
    <tag runat="server">
        <simpleProp>simpleValue</simpleProp>
    </tag>
      
  • Inner objects and controls with properties (Note:  the getter must construct the object, which unfortunately makes it impossible to specify a subclass in its place within the .aspx markup; null can't be returned)
    <tag runat="server">
        <innerObjProp innerProp="innerVal" innerProp2="innerVal2" />
    </tag>
     
  • Lists of controls when a List or List<T> getter is exposed (Note:  an IList<T> getter doesn't work and you can't return null from the getter; the setter is never called)
    <tag runat="server">
        <listProp>simpleValue</listProp>
    </tag>
       
  • A template containing 0+ controls via an ITemplate property; you need to instantiate the template in CreateChildControls() using template.InstantiateIn()
    <tag runat="server">
        <templateProp>Literal and <div runat="server">nested</div> content</templateProp>
    </tag>