An interesting article by Stuart Sierra on isa? usage. However, if he wants to argue against the usage of isa? with this article, I'm afraid I don't understand it. Please note that I'm not trying to say that isa? should be used everywhere, I'm just saying that the availability of instance? doesn't render isa? useless.
I'm actually specifically missing the point that usage of isa? in a piece of code is a smell of lack of understanding or fear of usage of multi-methods. I'm saying "smell", because depending on context, multi-methods might be an overkill.
Finally, I have a question to you guys here. Stuart writes: "Remember also that records and types are classes, not Vars, so to reference them from another namespace you must :import instead of :require them." This difference is ..., umm, let's say, not so tasteful. Is there any specific reason not to have another indirection over Vars? I guess there is probably a speed trade-off here, but in other areas, Clojure typically favors cleanliness over speed, too.