Rational ClearCase is used to store and version all source code files related to our 2.0 product and all client customized products based on 2.0. Rational ClearCase is integrated with Rational ClearQuest to track SDLC workflow and tie that workflow to changes in code.
Rational ClearCase is excellent for handling versioning and branching. No other tool I've used has the depth that ClearCase has when it comes to handling complex branching scenarios and identifying where certain versions of particular files are within a particular configuration.
Rational ClearCase handles parallel development of many dependent applications really well.
The use of ClearCase Views to switch between projects and configurations is extremely convenient as opposed to the local workstation model of the competitors.
ClearCase Mulitisite feels like a clunky solution for global development. The need to keep copies of the same VOBs in various location and keep them in sync by sending change packets across the network can be problematic at times. This systems seems to require a good bit of manual intevention when the VOBs miss a packet and get out of sync.
The learning curve can be overwhelming for new users.
The "Evil Twin" problem (where files with the same name are added to the same location, instead of versioning the same file) can be devestating to the system and can take days to fix. Unfortunately, I had this experience in my early days of ClearCase when somebody ran a script that inadvertantly created hundreds of "Evil Twin" files. The system was inaccesible for days as I worked to resolve this issue. It shouldn't be that easy to mess up the system so badly.
If development is centrallized to one location and your company releases hundreds of customized versions of your software per year, then ClearCase is the best tool for managing the complexity of multiple versions of customized software. If your company has globally distributed development, then I'd recommend Team Foundation Server over ClearCase. If your organization uses Agile Methodologies, then I'd recommend TFS with GIT.
Rational ClearCase is well suited for a company where development is done from one single location and there are a lot of dependent projects, systems or modules that need to be version controlled. Especially good for a company that creates customized software for clients. Well suited for disciplined development environments. Waterfall methodology mentality seems a better fit for ClearCase than Agile.
ClearCase is not well suited for companies that have development geographically distributed and using Agile Methodologies.