Let's not be too hard on Garmin as there are 100s if not 1000s of iPhone Apps that have not been upgraded to be compatible with iOS7. Since Apple is so secretive about their new products and software as well as when they might release them to the general public we all should cut some slack for these App vendors.
As we all should know, having a single computer program that provides navigational aids is a recipe for disaster, so having some redundancy is essential.
For some of us I suspect we have multiple Apple mobile devices... and given this it would make sense to only upgrade one of them to check things out before upgrading them all.
My sister in law tried upgrading her iPad in the UK over the weekend and she ran into all sorts of problems and ended up having her iPad unusable. Not sure what all she did, but she's now at her local Apple Store getting things sorted out.
As much as I like iOS7, it's not without issues, so my advice to others is to wait for 7.1 and check with other early adopters before leaping, so to speak.