Some of them are pretty cewl *like the sig you're currently using*
but most of them don't really look finished to me if you know what I mean...
Try to work a little on your placing of the text as that seems a little odd in some sigs and also try to blend the renders with the background a bit more, so it doesn't really look pasted on there if you know what I mean
(Sorry, didn't wanna sound harsh or anything, but that's just my opinion)