Last month, Rubicon contacted 460 randomly-selected iPhone users in the US and queried them about a broad range of issues, including satisfaction, usage patterns, and expenses. The company said that the survey has a margin of error of about plus or minus 3.8 points at the 90 percent confidence level.

According to their findings, half of the US iPhone users are under 30 and the majority of them, 75 percent, are prior Apple customers. They are using the iPhone mainly to check their emails and browse the web. About 60% said they browse the web on the iPhone daily and over 75 percent of iPhone users say they do a lot more mobile browsing on it than they did with their previous mobile phone. Unfortunately, it seems like about 40 percent of iPhone users say the product has trouble displaying some websites they want to visit. This is probably due at least in part to the absence of Adobe Flash on the iPhone.

In addition, according to Rubicon, it seems like the iPhone was a real winner for AT&T.  About half of iPhone users switched carriers to AT&T when they obtained the iPhone, Rubicon’s report concludes.

Still, Rubicon believes that the iPhone is far from being a mainstream phone. “The challenge for Apple is moving iPhone demand beyond early adopters and current Apple customers," said Michael Mace, a principal at Rubicon Consulting.

The good news for Apple is that their iPhone seems to be on the verge to replace the notebooks for its users. 28 percent of iPhone users surveyed said strongly that they often carry their iPhone instead of a notebook computer.

These findings confirm a previous study published last month by M:Metrics according to which 85 percent of iPhone owners are using the phone to browse the Web. At the same time, M:Metrics confirmed that the average iPhone user is male, aged 25-34, earns over $100,000 and has a college degree.