BlackBerry has native support for viewing Word, Excel, and PDF email attachments. If you want to create or edit documents there's a program called eOffice. I haven't used it but it gets 4 of 5 stars on Handango web site. Also, DataViz, the maker of Documents-to-Go says it's coming out with a BlackBerry version of D2G in "the first half of 2008."
I switched from a Palm Treo to BlackBerry several years ago, and you couldn't pay me enough to switch back.
Hope this helps!