What hardware do you have?
I use a I7-950 quad core clocked to 4Ghz: 6GB RAM and a SSD for the main Application launch. I never have any issues with zooming loading... however since my catalogs are on a slower HDD disk, if I load more than 50 pics at once, I hear constant churning of the HDD as it backs up the library I am working on. That to me is irritating. Sometimes I have 300 pics loaded, then it slows down even more... but used sanely, I have no real issues with it.
I have had about 4-5 blue screens of death caused by LR3 on my Win7 Ulti 64bit OS... in a 5 month period. But it was a non-issue since after rebooting, the libraries came up as they were before, I did not lose any work done in that session (perhaps that explains the constant churning of the non-SSD, harddrive).
Apart from the sharpness tool within LR3, I am pretty pleased with the product.