Andrew,
The 15mm is a truly great lens for what it handles best: very close quarters when other lenses won't cover it. I use it stopped down to 8 or 11 with the Voigtlander spirit level mounted side-by-side in the same make double shoe (which I use for my Leica 21 too) for architectural shots (my profession) on a tripod when my 21mm or Mamiya 7/43 can't cover the space. That is: for interior shots when there are three walls you want in the shot, but any other lens will just give you one corner at the most. I also use it recreationally, handheld outdoors, but always with the spirit level to depict space a bit differently and make use of that vast DOF. As with any very wide angle, you most often need some interest in the foreground and a level camera to get interesting results, at least in the urban landscape. If you accept some cropping, you can also use it as a "lazy-man" shift lens, just cropping the negative to eliminate too much sky, ground or side to get the "shifted" image with corrected perspective that you want.
In conclusion, I would say it is a great COMPLEMENT to other wide solutions, when they don't go wide enough. Optically I'd say it's well on par with my excellent Nikkor 20/2.8, although it needs some further stopping down to do justice without vignetting. Also, it's great fun, but if if your choice is between a lesser wide and the 15mm, you need to set your priorities.
Good luck (and do try one at least, holding it level!)
Richard