I believe that the more "explicit" way of doing implicit conversions is much better in terms of readability than the fully transparent one, at least in this example.

In my opinion, using implicits fully transparently from type A to type B is OK when you can always view an object of type A as being able to be used whenever and object of type B is needed. For example, the implicit conversion of a String to a RandomAccessSeq[Char] always makes sense - a String can always, conceptually, be viewed as a sequence of characters (in C, a string is just a sequence of characters, for example). A call to x.foreach(println) makes sense for all Strings.

On the other hand, the more explicit conversions should be used when an object of type A can sometimes be used as an object of type B. In your example, a call to foo("bar") does not makes sense and throws an error. As Scala does not have checked exceptions, a call to foo(s.toDate) clearly signals that there might be an exception thrown (s might not be a valid date). Also, foo("bar".toDate) clearly looks wrong, while you need to consult documentation to see why foo("bar") might be wrong. An example of this in the Scala standard library is conversions from Strings to Ints, via the toInt method of the RichString wrapper (Strings can be seen as Ints, but not all the time).