It all depends on what your definition of a DMS is. Plone can handle some document management tasks quite well, such as storing a large number of PDF's, making them findable and searchable.
It is not a good fit, in my experience, if your documents are actively being developed. If your content consists of spreadsheets or other office documents that people change daily, and that you want to check-in and check-out every time, there are specialist DMS's that do a better job.
For my use cases, which are 'intranet with some DM thrown in', Plone is a very good fit. If your use case is 100% document management, I would pick a specialist software in that field.
If it lies in the middle, well, your choice basically...
Alfresco (for instance) pretty much sucks as an intranet, but does the DM side very well. Plone is the reverse, it does the intranet side extremely well, handles relatively static PDF and Word content also well, but checking in XLSX fifty times a day per user is just a major hassle if that's your main objective.
So figure out where along the intranet to DMS spectrum you lie, and then pick a software that best serves those needs.
And (perhaps stating the glaringly obvious here), be aware that setting up a DMS so that it's workable is a far from trivial task, regardless of the software used...