A few months ago I purchased an M179 (based on a few recommendations)primarily to try the various patterns on acoustic guitar and harmony vocals I've only tried it so far on 3 applications:
Lead vocal (didn't like it - while fairly nuetral it really did nothing on the voice I tried it on)
Background vocals (both the omni and the figure 8 seemed to work well) - I had heard good things about this type of application, so while pleased with the results I was not surprised.
Acoustic guitar (I had mixed results) Obviously, as soon as you start trying to pick up a wider pattern and more "room sound" you become a victim of the room. I have a one room studio that serves as my "live" room for tracking and as my mixing environment (with a seperate room serving as a "vocal booth").
The first few times I recorded acoustic with a wider pattern I picked up too much "room" (and not in a good way). I then did some additional room treatment with panels of OC703 - which helped a lot. I'm still tweaking the room (moving panels around, etc) which is a major pain in the ass - but it should be worth the effort.
I don't know if I can specifically claim the tracks were "wider" but you can record room "reverb" (which may or may not be a good thing). I did feel I could get a more open sound (vs. a close mic'd sound)
That all being said - the M179 can be a useful mic with the wider patterns, but, a lot will depend on the room.
I just picked up
a Rode NT2A, which is also an adjustable mic. I have not used it a lot yet - but at this point I think I can safely say it absolutely kicks butt over the M179.