It's worth looking at the end user documentation (linked above by Torben) and the list of core and non-core packages (look for "packages" in the menu at the left of the user documentation page).
Then ask yourself if much of the functionality your business requires is substantially similar to the existing features and packages.
If it is, then you will almost definately get a better, cheaper and faster solution by going with OpenACS/.LRN because most of your work is done for you. Any extra functionality can be developed for you by OpenACS consultants.
