Here's a very unscientific and informal gauge of how great Wikipedia has become compared to other encyclopedias: the Google fight (www.googlefight.com). It simply compares Google hits and displays it in a rather humorous format.
Wikipedia - 11,200,000
Encyclopedia Britannica - 1,670,000 Encyclopaedia Britannica - 1,420,000 Encyclopædia Britannica - 803
Encyclopedia Americana - 94,700 Encyclopaedia Americana - 11,500 Encyclopædia Americana - 88
Encarta - 5,060,000 Microsoft Encarta - 1,070,000
Funk and Wagnalls - 36,400
Compton's Interactive Encyclopedia - 11,300
Grolier Multimedia Encyclopedia - 29,000 Grolier's Encyclopedia - 5,920
The Star Trek Encyclopedia - 108,000
Interesting to note that Wikipedia loses against "World Book" (14,800,000) but not "World Book Encyclopedia" (3,130,000).
For comparison, Wikipedia wins against Internet Explorer - 10,900,000 The Simpsons - 9,900,000 (the Simpson family member with the highest score is Lisa Simpson) iPod - 8,390,000 World Cup - 7,870,000 ESPN - 5,520,000 Warcraft - 3,860,000 Super Bowl - 3,590,000 Slashdot - 3,490,000 National Football League - 2,600,000 (although Wikipedia loses to "NFL" - 13,300,000) Pokemon - 2,470,000 (like "Encyclopædia Britannica", you get less results with the correct spelling of "Pokémon") Street Fighter - 1,900,000 (speaking of which, Marvel beats Capcom, which beats SNK) Starcraft - 1,680,000
Wikipedia tied against SourceForge, and lost embarassingly to football (42,700,000), Google (59,600,000) and eBay (65,300,000).
As for Wikipedia's dictionary counterpart, Wiktionary loses a lot of its Google fights...
Wiktionary - 331,000
Oxford English Dictionary - 2,090,000 The American Heritage Dictionary of the English Language - 1,160,000 New Oxford American Dictionary - 796,000 Concise Oxford Dictionary - 182,000
All data based on results on July 15, 2004.