Here is a good list of links about this.
The Horror Story from Simple Thoughts contains a list of issues and forum tickets related to this hot topic.
Robert Deaton of Something Unpredictable kindly crafted a
If you have upgraded to 2.0 without keeping a good copy of 1.5.2 then you'll need to go there to fetch an archived copy of older version.
You might also like to read CreepiGurl's experience here.
Note also from DESIGNLEGION that you must run the downgrading plugin under 2.0 After it's done successful then try overwriting 2.0 files with 1.5.2.
Here is another negative comment about accessibility of 2.0 by Mike Little here.
Last word, if you want challenges and sleepless nights go upgrading now. If not search around, learning from others, think it twice, set up experiemental blog, back up your databases, give it a try when you have nothing else more meaningful to do, think it over again for the third time....keep waiting.