I routinely use it if I'm going on an extended cross-country, but then I have ADS-B so am not reliant on SSR coverage. Hell, I even used it when the RV was registered RAAus and I was going interstate - though the number of times Controllers responded to my call of "Centre, good morning, RV8434" as
"Army 8434 good morning, go ahead" was enough for me to revert to Romeo8434 as that's what they saw from my ADS-B squawk.. Now the RV's VH- it doesn't matter.
I cant think of a time I've been knocked back on my initial request, though I've had it cancelled/suspended several times due lack of surveillance coverage however (
le P can explain better here) most times the Controller "kept" my plan in the system such that when I showed up on the ASD of the next sector,
they called
me and we picked up where we left off until I cancelled it on descent.
If you're going XC and going high, you'd be nuts
not to use it, it's free, you get traffic, the controllers seem to like being able to talk
directly to you (if you're in Class E) and you're
already in contact with Centre if you have a problem. There's no downside from what I can see.