I tend to see a loop pedal such as the Boss Loop Station as the prodigal son of the digital delay, however, many of the more up market digital delay pedals are now including a loop sampling feature.  Such pedals include the Boss DD-7, the Boss DD-20, the Akai Headrush and the Line 6 DL4 (limited to 14 seconds) and others.  The looping capacity of such pedals is typically a lot less than a dedicated looping pedal, in terms of both the available recording time and the ability to store loops, but of course a digital delay will have other modes of interesting delay effects available that a loop pedal might not have.

To my way of thinking, having a loop pedal feature in one of the most important effects a guitarist can have.  This is the case for learning the instrument, song writing and having the ability to create good solo instrumental performances.

In terms of learning the instrument,  playing over a backing, practising your improvisation over a backing and being able to listen back to your playing are essential ingredients.  You can get this by simply recording your playing or backing tracks, but to not have such a recording in a pedal (such as a digital delay with a loop sampling feature) makes the recording process difficult and such that it will constantly interrupt your practice.  Once you have a loop pedal, recording and play back becomes as versatile as the playing itself.

In terms of song writing, a loop pedal will allow you to hear the layering of bass, rhythm, … on the fly and can lead to a sort of improvisational song writing process where you can lay down and layer ideas with the tap of a foot as simply as playing them.

For solo instrumental performances, it can be quite hard to make a single guitar part sound full and alive, however with a loop sampler, you can really get the live effect of rhythm, lead and various sound effects interacting on the fly in a way that will hold a lot more interest to a listener.

If your considering a delay pedal and have not thought about the looping feature of these pedals, I encourage you to put some thought into just how much versatility and capability having a loop sampling feature will bring to your playing.