I spent a few years managing DB2 instances at a previous employer and I've seen it hold its own against mixed workloads (i.e. shitstorms of bad-behaved applications and people running ad-hoc queries) that I don't think even a well tuned PostgreSQL could handle today.
DB2 has some really annoying quirks(1) and that IBM'ish feel that's very hard to explain, but it's also very understandable (from a tuning perspective) for such a complex piece of software.
DB2 for Windows seemed mostly crap compared to Linux or AIX, though.
(1) For example, at least since I've last looked into it, DB2 had no decent MVCC(2) and it was a real pain to deal with all the lock-related problems that would arise from large write/update transactions running in parallel with small reads.
(2) At some point (9.5, IIRC) they tried to simulate MVCC by letting transactions read previous row versions from the transaction log. This seemed to come with its own set of significant drawbacks.