I have an 05 Mustang and the wheels just don't fill out the fenders like I'd like.  I think 25-30 mm spacers (or adapters as you may call them) would do the trick.  What are the implications on my car's health?  I promise I would buy the best ones available, and not skimp on crap.

PS I know buying a different offset wheel would do the trick but I like the ones I have more than any others.