Interested? I'm interested in everything to do with bags, so I read the glossies, check out places like this (way too often!) and generally lurk in bag departments. I am constantly curious and check out loads of things whether or not it is a designer I've heard of or own.
Loyalty? Not automatically to any brand, but there are definitely brands/houses whose collections I'm more interested in seeing, usually based on previous ownership (Anya Hindmarch for example) or simple aspiration and lust (Hermes! Just seen the Kelly 'flat' that folds into a clutch for the first time). Would never automatically go for, nor reject, anything though. But I do reject places that sell bags on the basis of poor customer service experience and stay loyal to those that are great which I know is a different question.
Decisions? Mood, season, bank balance, similarity to other bags I own, purpose - all sorts of variables. If I am experimenting though (don't do this often) with a new style or colour, I am much more likely to be very cautious and go for a mid-range bag. I don't think I'd risk buying a very colourful or unstructured bag (neither my natural style) from an expensive designer unless I was super sure it was for me.