I like saving 6 keystrokes, and in my opinion "fn" in place of "function" or "lambda" fully pays the price of admission, but I foresee some costs to if-seq/when-seq. Mainly the erosion of other good things about Clojure.
I like that seq is a simple idiom that appears in all contexts.
I like that I can think about conditional logic without worrying about complected transformations.
I like the appearance (even if it is a tiny bit false) that the sequence library is distinct from the language.
I am in awe of Clojure's existing bargains with truthiness.
To ameliorate my complaint about refactoring, "if-seq" could be called "if-let-seq" and a "let-seq" could be added, so adding/removing a condition would involve only the "if-" or "when-" prefix. But there you'd've lost 4 of the 6 saved keystrokes AND forgotten the lessons of letfn AND gone down a slippery slope toward an alternate universe of forms with "-seq" in their name.
Next, "some-seq", "remove-seq", "take-while-seq", etc., to similarly compose seq with their embedded test.
To redeem myself after all this negativity, I will say that if someone is eager to save me some keystrokes, I would love a Java var-args interop that didn't make me write out "maaaakkkkeee aaannnn arrraaaayyy ooffff ttthheee tttthhiiiinnngggsss".