i have about 8000 pics in my libarary and it works just fine. The only downtime you have is when you build 1:1 previews of a 200+ shot... that will take some time but its way faster now than before. the betaversion would use up all the memory i had for this operation, LR1 does it now with a third of the resources... i think best is when you import a new shoot to let the computer run by itself for a couple minutes, go have a coffee and come back afterwards. Also previews are discarded after a certain time (can be adjusted by user, default is 30days). so if you revisit a shot taken a year ago you will have to rebuilt the 1:1 previews.
I found my PC hanging and LR not responding way too frequent. Wonder if this is an operator error problem?
I P4 3.2 with 2 GB of RAM. HDD space is plenty. I did import about 40K images into LR though.