You really want a softphone? You know you'd rather play with asterisk and an IP phone or an FXS card. You could get yourself on TARP. Granted, hardware is expensive, but you don't need a hard phone to play with asterisk, when it comes down to it, so you wouldn't really have to pay any money if you didn't want to. Check out Ekiga or kiax. You'll have way more fun with asterisk+whatever than with just skype or gizmo.
Gizmo:
Pro:
* low barrier to entry
* If it talks SIP, it talks to Gizmo
* Development is ongoing, I've seen improvements recently
* ...?
Con:
* Not completely free (as in freedom)
* Linux version is behind.