I try to be as sensible as possible before I buy. I have a good collection of equipment, I can cover all my bases for work, but always keep my eye out for something new to add to my bag of tricks.
Recently I had my mind set on a TS-E 24mm 3.5LII, I knew it'd be great for landscapes and also be amazing for some creative portraits. I did all the research I usually do, found everyone agreed it was a beautiful piece of glass, some of the best optics on a canon lens, I started putting money to the side for it....then I had a play with it and I talked myself out of it. My heart told me I really wanted it, but my head so no. Don't get me wrong, it is a beautiful lens, I do love it and I still think I will get it one day, but the reality is that for my work (editorial/weddings/portraits/corporate events/travel) I wouldn't use it so often to justify the price. Instead, I need another wide aperture lens with AF, say a 35mm f1.4L, Sigma 85mm f1.4, or a canon 135mm f2L, all lenses I can use in many more situations. Cheaper as well which helps.
Any of you ever do the same thing, does sense kick in and you listen to your head or do you just go with your gut?