If the field value is just a single tiddler title, I'd skip putting in the square brackets, since they might get in the way later on if you want to pass the field value as a parameter to a widget or use it in a filter.
For example:
<$action-setfield $tiddler={{!!somefield}} text="foobar" />
or
<$list filter="[tag{!!somefield}}]">...</$list>
In both of these cases, if the square brackets were already part of the field value, then you wouldn't get the results you'd expect.
The first case would write to a tiddler that has literal square brackets in it's title, and the second case would look for a tag that had
square brackets in the tag value.
In any case, to make a link from a field value is pretty easy. All of the following will work:
<$link to={{!!somefield}}><$text text={{!!somefield}}/></$link>
or
<$tiddler tiddler={{!!somefield}}><$link/></$tiddler>
or even
enjoy,
-e