I still use my V4 mk2 regularly and it has not skipped a beat in nearly 3 years since I first got it, I am 22 though so hauling it isn't an issue.
It has had an outing on average once every 2 weeks to gigs and rehearsals, also gets frequent home use and still works perfectly for me so i've not witnessed reliability issues myself.
Valve Trace heads don't come up for sale very often, however. so anyone wanting one is in for a bit of a wait.