The once-reigning king of U.S. cell phones, Motorola’s Razr, is losing users in a big way to Apple’s iPhone. About a quarter of the current iPhone users, according to data from Rubicon Consulting, apparently switched from the Razr.

While Motorola seems to have taken the biggest hit from defectors, Windows Mobile and Blackberry owners left their handhelds behind in substantial numbers, too. 14 percent of the iPhone owners surveyed dumped Windows Mobile Devices, and 13 percent had been Blackberry users.

Only seven percent upgraded from a Palm device -- a figure that could indicate a high satisfaction rate with Palm’s handhelds, or that not many people are using the company’s products.

The loss of customers is bad news for Morotola. The company has held a strong position in the United States with about a quarter of the market, but only about five percent of the European cell phone market. Once the iPhone 3G hits store shelves on July 11, those number could drop even more.

The driving force behind the Razr wasn’t its feature set, according to ARCchart. The popular flip phone sports a look that users like, but ships with a ho-hum feature set that doesn’t stand out from the crowd. Apple’s iPhone, however, has the looks and the features consumers are looking for, despite a few limitations.

Apple’s iPhone, for example, doesn’t support multimedia text messages, doesn’t include a copy-and-paste feature, and doesn’t offer video chat support. It does, on the other hand, offer an easy to understand interface, can be software upgraded with little effort, and will be able to take advantage of a wide range of third-party applications once Apple opens its App Store in early July.

Apple’s competitors, including Motorola, have been scrambling to catch up with the original iPhone for a year. When iPhone 3G launches in July, the competition will be even stiffer, especially for the Razr.