It depends how much I am spending!
All three of the horses I have now did NOT have a pre-purchase exam. One was given to us by a friend (he's my dressage horse! :) ), Key was bought at an auction (he's my pleasure horse, and is fantastic! Going to the regional show this weekend!), and my weanling was bought from a different state (And is A-OK!).
It really depends! If you worked around the horse a lot, then you should know if he's alright. If you are questioning the horse's soundness, then you probably shouldn't get him in the first place! A pre-purchase exam never went amiss, but if the horse is more then 10,000, I'd say x-rays are a must. Sometimes, x-rays come up with something but the horse is totally fine!
Also, bring in a vet that your trainer DOES NOT use! Don't want the vet to be biased... :)