IBM is crowing about its increase in Lotus Notes licenses to 145 million, up five million in the past year. That's nice, but I'm willing to bet that Microsoft could issue a similar press release, and probably could claim even more Notes/Domino emigrants to Exchange.
In fact, for the past few years Microsoft has been doing exactly that.
If one looks to neutral analysts to be the line judge in this discussion, the water becomes even murkier, as eWeek points out:
Market share estimates vary widely for Exchange and Lotus Notes. Gartner Dataquest's most recent report from 2008 shows Notes narrowing the gap on market leader Exchange, with IBM's Notes owning 40 percent share worldwide and Microsoft grabbing 48 percent for Exchange.
IDC's annual market share analysis of collaborative environments puts Microsoft's market share at 52 percent, with IBM's market share slipping 5 percent to 37.7 percent. A Ferris Research survey of 917 organizations worldwide found Exchange in 65 percent of those shops.
In the land of the big incumbent software vendors, it's really a matter of customer ping-pong, as SAP and Oracle's back-and-forth suggests, without significant market share gains at each other's expense.
When open-source Zimbra/Yahoo! claims to have gained five million licenses at either IBM's or Microsoft's expense, that will be real news, because it will represent real market share gains for a competitor. But this sort of PR from IBM? It's really just saying, "We nabbed five million seats from Microsoft while it was stealing five million from us."