Like you mate i use 3.7.3, I'm happy with what i've got its taken me a good couple of months to get to where i am and how i want the forum, I'm not overly busy, but happy enough to say on this version.
I guess the big question is when will i upgrade??? well the answer has to be when theres enough jusification to upgrade, With all the hassle, with mods and template edits, The changeover has to provide me with enough reasons and improvement to make it all worth while.
Might still hang on until ver4 comes out then have a massive spring clean, and install some newer hacks and edits.
My 2 cents