Druapl 6.6 seems to work very nicely with PgSQL. I did not have any problems installing it nor using base modules.

You still have to be careful what modules you want to use and i would advise to install it on pgsql with all stuff enabled first and test for a while just to make sure you don't get into trouble

I'm not really sure if its such a great idea to move it to PgSQL as you wont really benefit from its powerful features. Modules don't use anything more than select,insert,update,delete,create/drop table. All modules should use abstraction layer for creating and dropping schema parts in .install files so you should be fine.

If you want to get rid of module and its tables and columns remember you have to disable module and then go to modules - uninstall.

I has some small issues with a few modules that we were testing on postgres. There were problems with smileys, phpbb2drupal, taxonomy_browser, advanced_forum and maybe some more i cant remember. But all of the problem were minor and you should be able to fix them with one or 2 queries adjusted in the module. The only problem is that you have to keep track of it or submit to the maintainer so you would have it in the next module releases. But its also possible through conditional sql based on the db type.

To summarize, it works, there are small issues but i would still not recommend. I love Postgres but in this environment? Why bother? Maybe convert to InnoDB if you have to ;-)