DB2 works logically no matter which edition you choose. Express-c and express work the same as the more expensive editions, and it is not hard for an expert to figure out which edition works well in a given situation.
The user community around DB2 is both robust and friendly. Consultants readily share their expertise and mentor more junior DBAs. Blogs, developerWorks, and forums all provide places to share content and ask questions.
IBM shares the details of how features are implemented and the logic behind new features. A wealth of information is available about each new release.
DB2 is stable on a variety of platforms. Implementing maintenance, recovery strategies, and high availability strategies is straight-forward and the options are well-documented.
DB2 product documentation is robust, and once you understand the structure, not hard to use.
DB2 DBAs are not as expensive as some other RDBMS DBAs.
Coming up with something that I dislike about DB2 is difficult. May of the things I might dislike are worse with other providers in my experience. Interacting with IBM support is frustrating for an experienced DB2 DBA, but not as bad for a newer DBA.
I have helped clients use DB2 in a vast array of environments - from the free DB2 Express-C all the way up to all the bells and whistles in Advanced Enterprise Edition. At all levels, DB2 is full-featured and any options that require separate licensing are easy to identify.
I particularly like DB2's High Availability solutions - particularly HADR is easy to use and work with. The cost of implementing it is low, and it doesn't require extensive expertise to set up and support.
I also have enjoyed working with BLU - setting up very high-speed analytics for small data warehouses is simple with BLU and the results are excellent.