I would certainly do it if I had 'formals'. I think sometimes, if you don't use the formal rooms very often, then you're paying to heat and cool basically empty square footage. It's simply a wasted space, that costs you money. Once upon a time, these rooms were used a lot more than today. Today, they're simply not used that often for most families. In my mind, rather than pay to maintain wasted square footage, I'd use it! I think it's a wonderful idea. I have several friends who've done this also...and I would in a heartbeat if I had 'formals'...but alas, I only have an extra bedroom :)
Debbie in Austin