i've been to one twice. first time was when i had a kernel panic about six months after i got chipper (macbook) and it took all night to figure out why it happened, thinking it was ram issues. so first thing in the morning, we went to the local apple store and found out that an app that went crazy was the source of the problem (it was some ipod conversion app, and i put in some bad files). we booted it on an external hard drive and got the bad files out and it was fast, about 30 minutes (because it was 11 am on a tuesday) and free of charge. i bought an external hard drive at a best buy the same day and not a kernel panic since then.
second time, it was alot slower because it was a saturday, and i was only confirming a problem with the battery which i suspected for some time beforehand and was seeing if i could have gotten one for free if the battery started acting crazy before the warranty went out (it didn't work), and i'll need to buy a new battery before it goes completely crazy.