Reminds me of the phrase "Nobody ever got fired for buying IBM." SharePoint works with so many MS products and there is such a huge eco system that goes with it. I think it's a safe bet for many companies that already rely on MS products. I would also give a lot of credit to the MS consulting/salesforce for many sales of that product.
edit: there are categories of software that have representatives that are more dysfunctional than Sharepoint. Still, Sharepoint is pretty much the worst document management/intranet package you can find.
Enterprise software can be surprisingly awful. blasdel's story is the best, but I remember being astonished that you could actually pay for version control software. The fact that said version control software didn't even have atomic commits but did, interestingly enough, run on its own proprietary file system was almost a Kafkaesque punchline.
You may consider it good, as long as you have never, ever used anything more sophisticated than a network share to manage your documents.