It's time (2016) to take another look at the 1889-CC $20. The PCGS Population Report shows totals for all of the CC-Mint Liberty Head Double Eagles. Where once the 1889-CC was considered "middle of the pack" in terms of overall rarity, it is now considered one of the most common dates among the CC Twenties -- the "most common" spot is now shared with the 1876-CC, the 1889-CC, and the 1890-CC. The 1884-CC and the 1875-CC come close, but most of the other CC-Mint Twenties have considerably smaller populations.
As plentiful as the 1889-CC may be, it is difficult to find nice ones. Most Mint State 1889-CC's are low-end Uncs., usually no better than MS62. PCGS has certified a small number of MS63 examples, but only a single MS64. The key to collecting this date is to seek examples that are as free of bagmarks as possible.