We have large commercial app that we want to convert from Delphi 6 to 2010. Approx 10 3rd party component sets, all with source code... I have heard warnings about Unicode
You'll find some useful answers in these StackOverflow questions:
Move project from Delphi 3 to Delphi 2010
When and Why Should I Use TStringBuilder?
Convert function to delphi 2009/2010 (unicode)
Unicode problems with Delphi 2009 / 2010 and windows API calls
Also, for what it is worth, I purchased Marco Cantu's Delphi 2009 Handbook. It was all I needed to make a relatively smooth converstion from Delphi 4 to Delphi 2009 in only a few weeks.
I do, however, recommend that you ensure your 3rd party packages have a Delphi 2009 upgrade, or you may have some real difficulties. Converting your own code is one thing. Converting someone else's is another.
I use two 3rd party packages, both with source code. Both had upgrades available, and the developer of one of them wrote that he had a lot of trouble upgrading his very complex component to the Unicode of Delphi 2009. It took him a few months, but he completed it. And as a result, I had little trouble with my implementation of his component when I did my upgrade.