My advice: do it!
I just flew from Frankfurt to San Francisco in business class and I may never be able to fly any other way ever again. When we were already eight hours into the 11 hour flight, I thought "Woah! Only three more hours! I better watch another movie!"
Why didn't I fly business class round trip? Let me break it down for you. Here's how the conversation went in order to get the upgrade. Keep in mind that I swear that I read on the website of a prominent German airline that it was 60,000 miles to upgrade a roundtrip flight to Europe.
"Hello? I'd like to upgrade my flight to Austria."
"Very good sir. I see that you have 60,000 miles. Which flight would you like to upgrade?"
"Uhhhhh... the whole one."
"I'm sorry sir, you only have 60,000 miles. You can only upgrade one flight."
"You could do the San Francisco to Munich flight, or the Munich to Vienna flight, or ..."
(Me, to myself .... "Oh, what they call 'flight' I call 'leg'...")
I've been saving those miles for five years now and even been using a credit card to accrue miles. In the end, I was disappointed that it really takes four times as many miles as I expected to get the upgrade for the round trip but I was happy to fly with enough leg room for a change.