I've used GIMP off and on for the past 5 years or so. After a long hiatus from making sigs, I recently started doing requests again. These first three where from requests, while the last one is just one I made for myself earlier today.
#1 has potential, there is some depth and ok lighting. You have sharpened too much, however.
Pen tool in #2 is so generic and ugly, you ruined it with that.
I like your text, so often it's too big, but yours is nice, and correctly placed in #1 and #4.