If you are on the latest BES release then you should be able to view attachments .doc, .xls, .pdf, .wav, .ppt, .mp3, .jpg. This is from memory, so don't blast me if I missed one
Sub-folders work fine with Exchange. Any subfolders created in Outlook will be available on the Blackberry, but you have to set each one to synch. By default only the main Inbox synchs.