I've been using 2.0 since it came out. The only problems that I have experienced on my MacPro 8-core are as follows:
1 - The random spinning beach ball during some operations that is annoying.
2 - When using the Tone Curve Target Button, the cursor does not revert back to a normal cursor. The only way that I found to turn the Target Button off is to click on another function like crop or clone and then click again on the same function. This restores the cursor that clicking on the image in the Develop module zooms in and out. I think that this is a bug.
3 - Not a problem with design, but a missing feature is soft proofing. With the cost of ink and media, soft proofing is an important feature to get an idea of what your print is going to look like when combining the selected paper profile and output sharpening.
Other than that, LR 2.0 is a great improvement over 1.4. Now I find myself using Photoshop CS3 less than 5% of the time for final prints.
North Wales, PAbudjamesphotography.com