MasterCard Inc. has long offered the ATM Hunter tool on its Web site, but interest skyrocketed when it released a version for smart phones.

"There has been a significant pickup in downloading the application and application use," says Cheryl Guerin, MasterCard's senior vice president for U.S. and digital marketing, though she would not say how many people are using it. The Purchase, N.Y., payments company released a version of ATM Hunter on Monday for Research in Motion Ltd.'s Blackberry smart phones, nearly a year after it began offering the tool for Apple Inc.'s iPhone.

The application uses phones' built-in GPS capabilities to locate nearby automated teller machines. Users can refine the search using several parameters, including ATMs owned by specific banks so that people can gain access to their accounts without paying a fee.

Guerin said MasterCard developed the service to increase cardholder loyalty, and perhaps encourage cardholders to promote the brand to others.

Some observers questioned the strategy. "That is optimistic at best," said Nick Holland, a senior analyst at Aite Group LLC, a Boston research company. "I am sure" MasterCard "will get branding applications out of this, but it's basically a mediocre application. There are good things about it, but the Apple store gives it two and a half stars out of five."

ATM Hunter users have posted mixed reviews about the service online and criticized it in particular for spotty service abroad.

One reviewer, identified as Scott-USA, said the iPhone app "is just OK for use in the U.S. and Canada, but for finding ATMs in other countries this app seems to be outdated and almost useless." The writer said he often visits Bangkok, where the app was unable to find any nearby ATM. Reviews at the Blackberry app store praised a rival service, offered by Allpoint, Cardtronics Inc.'s surcharge-free ATM network.

Reviewers gave the Allpoint ATM-finder service four stars out of five; MasterCard's ATM Hunter got one and a half stars.

Some reviews complained that ATM Hunter did not work in Ireland, Austria or the United Kingdom.

One reviewer, however, countered that it works on his Blackberry 9700 in the United Kingdom but is not foolproof. "It hasn't shown any Tesco [supermarket] ATMs, but it could be a lifesaver if you're in a new area or on holiday," the review said.

Guerin would not say how many ATMs are in the ATM Hunter database.