Need lens advice from the experts...

I am purchasing the Canon 6D in a few weeks.  My walk around lens will be the Canon 24-105.

I have a Sigma 15-30 EX Aspherical IF lens that I've used with my 40D.  Do any of you have any experience with this lens?  Will it good enough for the full frame 6D?  Will it be "almost as good" as my 24-105?  If not, would you suggest a replacement?

I want something that goes down to 14 or 15mm.  It doesn't have to be a zoom, nor do I want a Fish-eye.

Any help would be appreciated.  Thanks,

Howard