Which is the fastest iPhone carrier in your neighborhood?

A new app makes it easy (and fun!) to compare download speeds, but can it be trusted?

According to the snapshot at right, AT&T's (t) cellular service in Park Slope Brooklyn is nearly 5 times faster than Verizon's (vz) and more than 7 times faster than Sprint's (s).

At least that's the reading I get on my iPhone from my favorite armchair on a new app called CarrierCompare from SwayMarkets.

On my iPad running on AT&T's LTE network, the comparison is even starker: AT&T at 8.14 Mb/s is 12 times faster than Verizon and 19 times faster than Sprint.

As my colleague David Goldman reports on CNNMoney, CarrierCompare was released Friday to give you information the carriers have at their fingertips but have been keeping to themselves.

In theory the app will tell you exactly what you want to know: comparative speeds, responses and signal strengths with pinpoint accuracy for precisely where you live or work.

But as Goldman reports, there's a catch:

"The app is only as good as its crowd-sourced data. SwayMarkets has a starting data set pulled in from its previously released NetSnaps app, but CarrierCompare will only become really useful if a critical mass of people adopt it.

In other words, the AT&T data on my iPhone are probably accurate. Until more of my neighbors start using the app, the Verizon and Sprint data probably aren't.

The app currently runs only only on Apple (aapl) iOS devices, but a Google (goog) Android app is reportedly in the works.

Some users have found version 1.0 to be buggy. It worked for me. You can download it here. Free with ads.

All products and services featured are based solely on editorial selection. FORTUNE may receive compensation for some links to products and services on this website.

Quotes delayed at least 15 minutes. Market data provided by Interactive Data. ETF and Mutual Fund data provided by Morningstar, Inc. Dow Jones Terms & Conditions: S&P Index data is the property of Chicago Mercantile Exchange Inc. and its licensors. All rights reserved. Terms & Conditions. Powered and implemented by Interactive Data Managed Solutions