To Rick DeNatale on the list:

You really want a softphone? You know you'd rather play with asterisk and an IP phone or even an FXS card. You could get yourself on TARP.

Gizmo:

Pro:
* low barrier to entry
* If it talks SIP, it talks to Gizmo
* ...?

Con:
* Not completely free (as in freedom) 
* Linux version is behind.