I imported 5 years worth of files, one year at a time, covering a little over 27,000 images. The first 4 years went OK, with the occasional error (noted that the error isn't spelled out explicitly, which I hope Adobe can resolve over the next few releases, telling me it's borked without telling me why isn't of any use) although I assumed that LR was finding and ignoring duplicates (I had generated a lot of dupes).
The last year died about half way through with a large number of errors. So I imported the remaining 6 months worth of files one month at a time. *cough*.
The thing that really galled me was not being able to drag and drop out of grid view when building collections. I could only drag and drop out of the film strip, even though when highlighting images in the strip it also highlighted them in the grid. It felt more than a little counterintuitive.
Did anyone else notice drag and drop issues? I checked Adobe's docs, and it explicitly said drag and drop was supported in grid view.