okay here's the truth (personal experience) as compared to the opinions above on the dangers of spacers and adapters:
Most if not all failures in both instances are due to user error. People are stupid and lazy; thus dealing with the added preparation, work and maintenance surrounding these parts are usually overlooked.
Over the last 5 years i run and have run the following:
A) 1991 camaro z28 - 2.25" non-hub-centric adapters on all four corners. No failures (former daily driver/auto-x track slut).
B) 1987 monte carlo ss - 1.0" hub-centric adapters on the rear. No failures (with m&h drag radials running 12.5 @ 116mph on nitrous).
C) 2007 trailblazer ss - .5" hub-centric spacers front, 1.0" hub-centric adapters rear. No failures (we use et (extended tip) lug nuts).
D) 2002 camaro b4c - 1.0" non-hub-centric adapters front, 1.5" non-hub-centric adapters rear. No failures (this is our spring, summer & fall daily driver, and the wife's auto-x car).
E) 1987 buick turbo t - custom width (l&r) non-hub-centric spacers on the rear. No failures (my auto-x car and this includes transbrake launches at the drag strip).
So there you have it. Everything we have owned/driven in the last 5 years has had some form of wheel adapter or spacer on it. Hell, even gm from the factory has run shim spacers (80's f-bodies) and some quality wheel manufacturers include them with their products. Pay attention to your lug nut torque settings and you'll be fine (that means you'll have to take the wheels off to check the adapters after xx miles of driving just like you would a wheel).
A small price to pay for improved function and form, imo.