Made another Boulevardier last night, incorporating some of the suggestions in this thread (1.5:1:1 rye, cherry garnish, on rocks rather than up).
The rye was certainly very prominent, but overall a nicely balanced drink. Personally I'd maybe up the Campari very slightly while keeping the rye:vermouth ratio the same.
Of the Campari alternatives listed above, I've only tried Aperol. I find it lacks the bitterness I like in a Negroni (not tried in a Boulevardier), but I do like an Aperol Spritz. Gran Classico seems impossible to get in the UK, but will definitely pick up a couple of the others.