T-Mobile (NASDAQ:TMUS) has made major strides over the last two years.

The company has passed Sprint (NYSE:S) to become the third-biggest wireless carrier and it has arguably forced the entire industry to change how it does business. You could fairly credit the Un-carrier causing Sprint and Verizon (NYSE:VZ) to end contracts, and the company deserves at least some thanks for driving prices down.

Along with those business changes, T-Mobile has also invested in improving its network. For years that was the company's Achilles Heel -- the reason why people could not jump from Verizon or AT&T (NYSE:T). For many users, specifically those in major metropolitan areas, the improvements have made switching to T-Mobile viable. 

Numbers from the latest RootMetrics Mobile Network Performance in the U.S. survey for the first half of 2015 show that these improvements are real and that T-Mobile has made great strides. They also detail a major weakness in the company's coverage, which may give certain potential customers pause.

T-Mobile is in the ballpark

RootMetrics' first half of 2015 overall performance rankings. Source: RootMetrics.

While T-Mobile's network has slipped slightly in the overall ratings since the second half of 2014, it still ranks above where the top networks were ranked in the second half of 2013 survey.

2013 second half survey. Source: RootMetrics.

As you can see above, T-Mobile has not only climbed, it has surpassed where Verizon was as the leader just 18 months ago. It's clear the company has improved its network, but its competitors have gotten stronger as well.

Where is T-Mobile lacking?
RootMetrics praised T-Mobile in its report but also pointed out that the company has some glaring holes in its coverage, which lowers its overall score:

T-Mobile was again shut out of United States RootScore Awards, but the network finished a strong third for network speed and data performance. We've noted before that T-Mobile typically performs much better in metro areas than it does at state or national levels, and this was indeed the case in the first half of 2015.

As our Metro RootScore overview below shows, T-Mobile's performance within metro areas was strong in multiple test categories, with improved data reliability and fast speeds. If you primarily use your smartphone in a major urban environment, T-Mobile remains a solid choice. Even though urban areas carry more weight in our results, T-Mobile currently lacks the broad coverage to excel in our National or State RootScore studies.

This is not an accident. T-Mobile has focused on improving service in major metro areas and has ignored less-populated regions in some cases and paid them only limited attention in others.

Why is this bad for T-Mobile?
The company deserves credit for the big gains it has made in such a short time. Still, it has not improved at the same level as its closest rival, Sprint, and the lack of coverage in less-populated areas makes choosing the company difficult for anyone who regularly travels outside of major cities.

"T-Mobile made a strong showing in metro testing," RootMetrics stated in its report. "However, at the state level, the network experienced difficulties. This is a trend we've witnessed before: T-Mobile continues to improve within the metros we test, but its network upgrades have yet to translate into significant performance gains at state or national levels."

Currently, T-Mobile has made itself a viable choice for wireless customers who live in the areas where it has strong coverage and who rarely travel. The company scored especially well on RootMetrics' speed tests and the survey company suggests the results may suggest big improvements are possible going forward:

In addition to the fast speeds we found from T-Mobile, the network improved its results in data reliability testing. Getting connected and staying connected are at the core of a positive consumer experience, and when you add fast speeds to the mix, the results could be a game changer. We'll definitely have our eye on T-Mobile's data reliability results in the coming months.

These results are positive, but the T-Mobile network is still holding the company back. The Un-carrier has made itself attractive to consumers with price, service, and transparency, but it needs to widen its network and improve service over a larger swath of the U.S. before it can be considered a real option for everyone.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.