mine is dedicated room too I'm afraid.. yes it's more expensive but on the plus side, the extension is adding money to the value of the place. Even with a garage conversion I'd imagine people would rather have a useable room than a garage to put their cars in. I think HC is the new car tinkering hobby.
I think there are a few plus sides to having a dedicated room.
One, as I said I think an extension will add value to your house, even a converted garage.
Two, with a duel purpose lounge you will have speakers and possibly wire and memorabillia and a big screen about etc and often I think this just doesn't work as well as a dedicated room. Often it looks cramped and a bit like boys toys gone mad.. and I've been as guilty of that as anyone..
Three. You can design your dedicated room to look cool, you can hide all your wires properly, you can set it up too look like a luxurious cinema/listening room and not feel bad that your stuff is everyone messing up the lounge.
Four, the pure convenience of having a room you can listen or watch stuff without bothering people. It's like having a shed or allotment in the old days, you can just dissapear and enjoy everything in peace. And when you have parties it's perfect!
I'm going to post pics of mine as I started a few weeks ago when the place was being built. It's not the biggest, 10ft by 22ft, but I absolutely love it.
And I love having the option of being able to escape to watch a film or listen to music whenever I want to in a really, really brilliant room :-)
As you can tell, I've just finished my room and after a year of it being planned and built, I am incredibly happy with it