This is a welcome addition to the limited number of applications available on the Mac platform that enable automatic notification of prayer times for Muslim users.
I have been using Athan Pro v3.6 on my PC (http:// www.islamicfinder.org) and have found that their calculation have been close to what my local prayer times should be with only a few adjustments here and there. The application provide comprehensive means to adjust the settings from the latitudes/longitudes, methods, location, athans sounds etc. and I have found it to be invaluable to my needs and I believe others.
As a comparison, Guidance 1.3 also provide most of the tweaks and adjustment options available in Athan Pro but I found that some of the options are not as detailed or a bit limited. For example, minute adjustment in Guidance only goes to 15 minutes each way (30 minutes ± in Athan Pro), the option of playing the Du'a/suplication after Athan is not available and the location database is limited (and not easily adjustable to fine tune).
In the other hand, Guidance ability to pause iTunes when Athan is playing or the inclusion of Growl notification (much more better than Windows balloon in my opinion) gives it advantage over the Windows offering.
However, the one most important thing for this kind of application is the ability to provide prayer times to match as closely as possible to your local mosque's published prayer times (as an authoritative figure) but unfortunately, Guidance is far from close on all method of calculations in determining Fajr and Isha' prayer times (the usual Achilles' heel on this type of application). For example, for Manchester, Greater Manchester, UK on June 1st, 2009 with ISNA as the method and standard/default settings on the rest, IslamicFinder.org (Athan Pro) gives you Fajr/Isha' time of 02h31/23h42 compared to Guidance's 01h21/01h06 respectively. As you can see the differences are big enough to warrant a careful observation of the times given by Guidance.
Perhaps, in the future the developer could offer a much more detail adjustment or choices to the settings but for now I think this application is much better than the rest of the offerings on the Mac platform in term of features and usage and if you find you are using this application, do watch for the prayer times given and adjust or make note accordingly. With no cost whatsoever to the end user (donation is encouraged or the very least, make a prayer for them for their good deeds), this application is promising and will be invaluable to Muslim users anywhere they are. May Allah give rewards to them for enabling us to observe our daily prayer times on our Mac.