You know the line about never getting a second chance to make a first impression? Luckily for Microsoft Corp., it turns out not to be true.

Windows Phone 7, Microsoft’s attempt to claw its way back into the lucrative smartphone arena, was released a year ago to respectful but hardly adulatory reviews. It was good, but not good enough to go head-to-head with the big boys, Apple Inc. and Google Inc.

Now it is. Microsoft is rolling out Windows Phone 7.5, also known as “Mango,” an altogether more polished and potent contender.

Unlike Apple’s iOS, Google’s Android and just about every other mobile-device operating system, a Windows phone doesn’t present you with rows of icons representing apps. Instead, the home screen consists of a number of colorful tiles that represent the phone’s key functions. These tiles can update themselves with new information, so that, for example, you can see your next appointment at a glance, or how many new emails you’ve received.

Some of the tiles lead to what Microsoft calls hubs — collections of information and apps organized around a theme. The People hub, for example, aggregates your contacts and social-network feeds, providing one-stop shopping for getting and staying in touch with people important to you.

One of the cleverest features in the new version of the operating system is the way it integrates the search function with the ability to discover and incorporate new apps.

For example, I used my test phone, a Samsung Focus, to find Italian restaurants near Bloomberg’s San Francisco offices. The software not only gave me the usual reviews and directions, but also suggested I might want to download the OpenTable app for booking reservations online. Once I did, future restaurant searches gave me a reservation option that automatically launched the app.

Microsoft says its Marketplace now has 30,000 apps, far fewer than the 500,000 or so available for iPhone or 250,000 for Android. At least Microsoft, like Apple and unlike Google, vets new apps before they go into the Marketplace,.