I've been using is.gd for a couple of reasons. First, "is.gd" is nice and short, and I find it extremely easy to type. The flow from key to key is very smooth. After that, the is.gd home page loads very quickly.

However, I find that the biggest advantage of is.gd , in comparison to bit.ly, cli.gs, TinyURL, etc, is the initial placement of the cursor.

On is.gd's home page, the cursor is placed in the text field, so all you need to do is paste your long URL, and hit enter. On the resulting page, the compressed URL is highlighted for you! So all you need to do is hit CMD+C/CTRL+C, and voila, the compressed URL is in your clipboard.