I am rolling out my group to use CS4 but we have encountered one significant bug/feature that is slowing the process down for us.
We produce product catalogs where our SKU numbers/Item numbers are mixed alpha numeric listings. We will have hundreds HDI106 type entries on a page. In CS3 the spellcheck ignored this or somehow was enabled to ignore such type of entries.
With CS4 it is not and spellchecking is becoming very slow to go over a document. We could build dictionary of all our SKU numbers, but we have many sections and many users working on our catalogs.