Given the known issues with the kestrel w Horus ( the positive aspects of it aside, as there are some), that being battery leaks and data dump when changing batteries and the in ability to update the software, I found a PDA with horus ATRAG and a separate windmeter (weatherhawk WM 350) to be the better option. Yes, it is more items and bulkier but I also found using the ATRAG easier and less cumbersome to use on a PDA compared to the kestrel version as well as the iphone/ipad apps such as AE, shooter istrelok, mildot, etc.
For all the features that some of the apps have, they are not intuitive and or user friendly. Meaning you have to go find out how to use or configure them to get the data you want and even then they sometimes dump your settings. The user interfaces and bells and whistles are eye catching but compared to using ATRAG on a PDA, the PDA is just easy. You get a physical manual (or you can download one) to explain everything and really, if you watch the 2 videos that Todd Hodnett does on how to use it, you can pick up a PDA and make it work. FFS and Coldbore are also PDA programs worth considering but they too have interfaces that are a bit more involved but they do allow you to tweak more than others, they also cost more than apps but they are in a different league then apps so it's not an apples to apples comparison looking at price alone. I like AE ,shooter and all the other apps but given the dicking around they require with them, I use them as back ups to my PDA and just use them for the moat minimal information.
Honestly, AE gets good update support from the writer but for the love of God, if he would only write a real manual that people could down load instead of the ramblings on the webpage, it might be easier to use out of the gate.
I know someone will chime in on how easy the apps are and yada yada yada and I get it, I can use them, it's not rocket science, but I find them frustrating to work with compared to a PDA. So my recommendation is go with a PDA with ATRAG/ FFS/coldbore and don't waste too much time dicking around with the apps. If you do a price break out of the cost involved for a kestrel with ATRAG vs a standalone windmeter, PDA and a copy of ATRAG, you will find you can get a used PDA and a current and updatable version of ATRAG for close to the same cost. This is just my opinion based on my experiences and I'm sure someone will vehemently object to something I said here but hey, it's the internet.