I think that it really depends on the size of your team(s), and what you want out of source control.
I used bugzilla in combination with Perforce for a couple of years and found that both were really very good at their own individual things while working in a very small team (2-3 people), but the suffered from a lack of integration between them and from some little idiosyncrasies that took time to get used to.
I recently moved to a new job where TFS is used extensively. There are 4 main teams in this company with 10-12 developers in each, split into further project teams below that level, and it is in this kind of environment that TFS really shines imo. It's biggest advantages in my view are:
1) The integration with Visual Studio - it's not just a case of having less windows open, but it really does speed things up and make your life easier. Things like VS automatically checking out files for you as you work (no issues with accidental checkouts due to lockless editing), being able to synronise local + TFs builds, being able to quickly compare the local version against previous ones..yes you can get 3rd party plugins to integrate but none to this level and with the same stability.
2) The communication features - simple things like integraton with Live Messenger (provided you configure TFS correctly) are great for large teams. We use WLM to communicate accross the office and for collaboration as its just quicker than walking over to someone else every time you need to ask a quick question.
3) Linking builds/changelists to tasks - Yes other SCMs do this but again it's just done in a very nice, integrated fashion..I guess it's nothing special to TFS but personally I like how it tracks this.
4) Ease of merging/lockless editing. I've had experience with some other merge tools and the TFS one works nicely enough, making merging after concurrent editing pretty simple. It's very similar to perforce in this respect, but also with a usually pretty effective auto-merge tool which I use for tiny edits that I know cannot cause any potential issues with edits other developers are working on.
5) Auto building/build management. Working with a couple of large solutions containing 20-30 projects that depend on each other, this is a godsend. We have it set to queue up a build every 20 minutes IF something has changed, and when one has happened its listed in the history log..so easy to see when you need to update your local libraries.
I don't have any experience with configuring it other than build management, but I have heard that this is the worst part of TFS..that its a bit of a pain to get everything running correctly.
So, translating that to a business case..I'd say that if you are a Microsoft software house with large/multiple teams, then the time savings and productivity improvements that you will see as a result of the above features are worth the investment in setting it up. Its free to use in most cases as you will probably have a MSDN subscription (maybe some CAL issues but i'm not sure) so your biggest cost will be in user training and configuration.